Form 424B5 INTERSTATE POWER & LIGHT

Filed Pursuant to Rule 424(b)(5)
Registration No. 333-222076-02

The information in this preliminary prospectus supplement is not complete and may be changed. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell these securities and are not soliciting offers to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to Completion

Preliminary Prospectus Supplement dated September 19, 2018

Prospectus Supplement

(To Prospectus dated December 15, 2017)

$            

Interstate Power and Light Company

    % Senior Debentures due 20    

We will pay interest on the senior debentures semi-annually in arrears on              and             of each year, beginning on                     , 2019. The senior debentures will mature on                     , 20     . We may redeem some or all of the senior debentures at any time and from time to time at the applicable redemption price described in this prospectus supplement.

The senior debentures will be our unsecured senior obligations and rank equally with our other unsecured senior indebtedness from time to time outstanding. The senior debentures will be issued only in registered form in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof. The senior debentures are a new issue of securities with no established trading market. We currently have no intention to apply to list the senior debentures on any securities exchange or to seek their admission to trading on any automated quotation system.

Investing in the senior debentures involves risks. See “Risk Factors” beginning on page S-5 of this prospectus supplement and page 14 of our Annual Report on Form 10-K for the year ended December 31, 2017, as such discussion may be amended or updated in other reports filed by us with the U.S. Securities and Exchange Commission, or the SEC. The Risk Factors included in our Annual Report on Form 10-K, as amended or updated, are incorporated by reference herein.

Neither the SEC nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The senior debentures will be available for delivery in book-entry form only through The Depository Trust Company, or DTC, on or about                     , 2018.

Joint Book-Running Managers

Co-Managers

The date of this prospectus supplement is                     , 2018.

TABLE OF CONTENTS

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ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering. The second part, the accompanying prospectus, gives more general information, some of which may not apply to this offering. You should read the entire prospectus supplement, as well as the accompanying prospectus and the documents incorporated by reference that are described under “Where You Can Find More Information” in the accompanying prospectus. Some of these documents, however, are filed on a combined basis with our parent, Alliant Energy Corporation, and its direct subsidiary, Wisconsin Power and Light Company. Information contained in these documents relating to those entities is filed by them on their own behalf and not by us and is not incorporated by reference in this prospectus supplement or the accompanying prospectus. The senior debentures are not obligations of, or guaranteed by, Alliant Energy Corporation or Wisconsin Power and Light Company and you should not rely on that information when deciding whether to invest in our senior debentures. In the event that the description of the offering varies between this prospectus supplement and the accompanying prospectus, you should rely on the information contained in this prospectus supplement.

You should rely only on the information relating solely to Interstate Power and Light Company contained or incorporated by reference in this prospectus supplement, the accompanying prospectus and any related free writing prospectus prepared by or on behalf of us or to which we have referred you. We have not, and the underwriters have not, authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not, and the underwriters are not, making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference is accurate only as of the respective dates of those documents in which the information is contained. Our business, financial condition, results of operations and prospects may have changed since any such date.

Unless we otherwise indicate or unless the context requires otherwise, all references in this prospectus supplement to “we,” “our,” “us” or similar references mean Interstate Power and Light Company.

Our principal executive offices are located at Alliant Energy Tower, 200 First Street, SE, Cedar Rapids, Iowa 52401, and our telephone number is (319) 786-4411.

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FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the information incorporated by reference herein or therein contain forward-looking statements intended to qualify for the safe harbors from liability established by the Private Securities Litigation Reform Act of 1995. All statements, other than statements of historical fact, included in this prospectus supplement or in the accompanying prospectus or incorporated by reference herein or therein, including statements regarding anticipated financial performance, business strategy and management’s plans and objectives for future operations, are forward-looking statements. These forward-looking statements can be identified as such because the statements generally include words such as “may,” “believe,” “expect,” “anticipate,” “plan,” “project, “ “will,” “projections,” “estimate,” or other similar expressions. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from those expressed in, or implied by, these statements. Readers are cautioned not to place undue reliance on these forward-looking statements. All forward-looking statements included in this prospectus supplement, the accompanying prospectus or in any document incorporated by reference herein or therein speak only as of the date of this prospectus supplement, the accompanying prospectus or the document incorporated by reference, as the case may be. Additional information concerning factors that could cause actual results to differ materially from those in the forward-looking statements is contained under “Risk Factors” on page S-5 of this prospectus supplement, page 4 of the accompanying prospectus and in other documents that we file from time to time with the SEC that are incorporated by reference into this prospectus supplement and the accompanying prospectus, including, but not limited to, the risk factor disclosure beginning on page 14 of our Annual Report on Form 10-K for the fiscal year ended December 31, 2017. Some, but not all, of the risks and uncertainties that could materially affect actual results include the following:

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We assume no obligation, and disclaim any duty, to update the forward-looking statements in this prospectus, except as required by law.

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S-2

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S-4

RISK FACTORS

Investing in the senior debentures involves risk. You should carefully consider the following risk factors together with all the other information contained in this prospectus supplement or the accompanying prospectus or incorporated by reference in this prospectus supplement or the accompanying prospectus. You should also consider the risks and uncertainties discussed under the caption “Risk Factors” included in our Annual Report on Form 10-K for the year ended December 31, 2017, which is incorporated by reference in this prospectus supplement and the accompanying prospectus, and which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future.

Absence of market for the senior debentures - The senior debentures are a new issue of securities with no established trading market. We currently have no intention to apply to list the senior debentures on any securities exchange or to seek their admission to trading on any automated quotation system. Accordingly, we cannot provide any assurance as to the development or liquidity of any market for the senior debentures. See “Underwriting.” Furthermore, the market price for the senior debentures may be adversely impacted by fluctuations in interest rates.

The senior debentures may not be a suitable investment for all investors seeking exposure to green assets - While we intend to use an amount equal to or in excess of the net proceeds of this offering for construction and development of Eligible Green Projects as described under the caption “Use of Proceeds,” there can be no assurance that the expenditures funded with the proceeds from the senior debentures will meet every potential investor’s expectations regarding environmental sustainability or performance. Adverse environmental or social impacts may occur during the design, construction and operation of the projects or the projects may become controversial or criticized by activist groups or other stakeholders.

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USE OF PROCEEDS

We expect to receive net proceeds from this offering of approximately $             million, after deducting the underwriting discount and estimated offering expenses payable by us. An amount equal to or in excess of the net proceeds from this offering will be or have been used for construction and development of wind and solar electric generating facilities, or Eligible Green Projects.

Eligible Green Projects may include both projects with disbursements made during the 24 months preceding the issue date of the senior debentures and projects with disbursements to be made at any time following the issue date of the senior debentures up to the maturity date of the senior debentures.

Management of Net Proceeds

Since a portion of the disbursements for Eligible Green Projects to be made from the net proceeds from this offering have not yet been made, until such disbursements are made, we intend to use the proceeds to reduce outstanding commercial paper or reduce outstanding capital under our receivables purchase and sale program, or we may place the proceeds in short-term instruments. As of August 31, 2018, our outstanding commercial paper classified as long-term debt had an interest rate of 2.23% and a remaining maturity of six days. As of August 31, 2018, our outstanding receivables purchase and sale program had an annualized commercial paper yield rate of 2.29%. Payment of principal and interest on the senior debentures will be made from our general funds and will not be directly linked to the performance of any Eligible Green Project. We intend to make disbursements of an amount equal or in excess of to the net proceeds from this offering for Eligible Green Projects within six months of the date of issuance of the senior debentures.

Reporting

Within one year of the issuance of the senior debentures and annually until the disbursement of an amount equal to or in excess of the net proceeds from this offering for Eligible Green Projects, we will provide the following information on Alliant Energy Corporation’s corporate website:

We will also provide on Alliant Energy Corporation’s corporate website (1) a report with an assertion by management regarding the amounts disbursed for Eligible Green Projects during the reporting period and (2) an attestation report from an independent accountant in respect of the independent accountant’s examination of management’s assertion conducted in accordance with attestation standards established by the American Institute of Certified Public Accountants.

Information contained on our website or any website of Alliant Energy Corporation is not and should not be deemed a part of this prospectus supplement, the accompanying prospectus or any other documents incorporated by reference herein and therein.

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CAPITALIZATION

The following table sets forth our consolidated capitalization as of June 30, 2018 on an actual basis and as adjusted to give effect to this offering, the anticipated use of the net proceeds from this offering as described under “Use of Proceeds.”

S-7

THE COMPANY

We are a public utility primarily serving retail customers in Iowa. We are engaged principally in:

As of December 31, 2017, we served approximately 490,000 retail electric customers and 220,000 retail natural gas customers.

All of our common stock is owned by Alliant Energy Corporation, a regulated investor-owned public utility holding company with subsidiaries, including us, serving primarily electricity and natural gas customers in the Midwest.

S-8

DESCRIPTION OF SENIOR DEBENTURES

We have summarized provisions of the senior debentures below. This summary supplements and, to the extent inconsistent with, replaces the description of the general terms and provisions of the debt securities under the caption “Description of Debt Securities” in the accompanying prospectus. We will issue the senior debentures as a separate series of securities under an indenture, dated as of August 20, 2003, between us and The Bank of New York Mellon Trust Company, N.A., as successor trustee. The indenture is described in the accompanying prospectus.

General

The indenture does not limit the aggregate principal amount of senior unsecured debt securities that we may issue under it, and provides that we may issue, without the consent of holders of the senior debentures, securities under the indenture from time to time in one or more series pursuant to the terms of one or more supplemental indentures, board resolutions or officer’s certificates creating the series. The indenture does not give holders of the senior debentures protection in the event we engage in a highly leveraged or other transaction that may adversely affect holders of the senior debentures. As of the date of this prospectus supplement, we have $250.0 million aggregate principal amount of 7.25% senior debentures due 2018, $200.0 million aggregate principal amount of 3.65% senior debentures due 2020, $500.0 million aggregate principal amount of 3.25% senior debentures due 2024, $50.0 million aggregate principal amount of 5.50% senior debentures due 2025, $250.0 million aggregate principal amount of 3.40% senior debentures due 2025, $100.0 million aggregate principal amount of 6.45% senior debentures due 2033, $125.0 million aggregate principal amount of 6.30% senior debentures due 2034, $300.0 million aggregate principal amount of 6.25% senior debentures due 2039, $250.0 million aggregate principal amount of 4.70% senior debentures due 2043 and $300.0 million aggregate principal amount of 3.70% senior debentures due 2046 outstanding under the indenture.

We are initially offering the senior debentures in the aggregate principal amount of $            million. We may, without the consent of the holders of the senior debentures, issue additional senior debentures in the future having the same ranking, interest rate, maturity and other terms, except for the public offering price, issue date and, if applicable, the initial interest payment date as the senior debentures we offer by this prospectus supplement. Any such additional senior debentures having such similar terms, together with the senior debentures, may constitute a single series of senior debentures under the indenture.

No sinking fund will be established for the benefit of the senior debentures.

Maturity and Interest

The senior debentures will mature on                 , 20    . Each senior debenture will bear interest from                 , 2018, or from and including the most recent interest payment date to which we have paid interest, at the rate of     % per year. We will pay interest semi-annually in arrears, on                  and                 , commencing                 , 2019, to the persons in whose names the senior debentures are registered at the close of business on the fifteenth calendar day (whether or not a business day) before each interest payment date. Interest will be computed on the basis of a 360-day year comprised of twelve 30-day months. In the event that any interest payment date does not fall on a business day, interest on the senior debentures will be paid on the immediately succeeding business day, without any additional interest paid as a result of such delay.

Ranking

The senior debentures will be our senior, unsecured and unsubordinated obligations, ranking equally and ratably with all our other senior, unsecured and unsubordinated obligations from time to time outstanding. The senior debentures will be effectively subordinated to all of our secured indebtedness to the extent of the assets securing such indebtedness. We do not currently have any secured indebtedness.

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As of June 30, 2018, giving pro forma effect to this offering and our expected use of the net proceeds of the offering, we would have had $            million aggregate principal amount of unsecured long-term debt outstanding (excluding current portion).

Optional Redemption

At any time or from time to time prior to                 , 20     , or the Par Call Date, the senior debentures will be redeemable as a whole or in part, at our option, at a redemption price equal to the greater of (i) 100% of the principal amount of such senior debentures and (ii) the sum, as determined by the Independent Investment Banker and delivered to the trustee, of the present values of the remaining scheduled payments of principal and interest thereon that would be due if the senior debentures matured on the Par Call Date (exclusive of interest accrued to the date of redemption) discounted to the redemption date on a semiannual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Rate plus                     basis points, plus in each case accrued and unpaid interest, if any, to, but excluding, the redemption date. At any time on or after                 , 20    , the senior debentures will be redeemable as a whole or in part, at our option, at a redemption price equal to 100% of the principal amount of the senior debentures being redeemed plus accrued and unpaid interest, if any, to, but excluding, the redemption date.

“Comparable Treasury Issue” means the United States Treasury security or securities selected by an Independent Investment Banker as having an actual or interpolated maturity comparable to the remaining term of the senior debentures (assuming for this purpose that the senior debentures matured on the applicable Par Call Date) to be redeemed that would be utilized, at the time of selection and in accordance with customary financial practice, in pricing new issues of corporate debt securities of a comparable maturity to the remaining term of such senior debentures.

“Comparable Treasury Price” means, with respect to any redemption date, (A) the average of the Reference Treasury Dealer Quotations for such redemption date, after excluding the highest and lowest such Reference Treasury Dealer Quotations, or (B) if the Independent Investment Banker obtains fewer than four such Reference Treasury Dealer Quotations, the average of all such quotations.

“Independent Investment Banker” means one of the Reference Treasury Dealers appointed by us.

“Reference Treasury Dealer” means each of (i) Goldman Sachs & Co. LLC and Wells Fargo Securities, LLC, and their respective successors, (ii) a primary U.S. Government securities dealer located in the United States (a “Primary Treasury Dealer”) selected by MUFG Securities Americas Inc. or one of its affiliates, and (iii) a Primary Treasury Dealer selected by us, and such Primary Treasury Dealers’ respective successors that we specify from time to time; provided, however, that if any of the foregoing or their affiliates cease to be a Primary Treasury Dealer, we will substitute therefor another Primary Treasury Dealer.

“Reference Treasury Dealer Quotations” means, with respect to each Reference Treasury Dealer and any redemption date, the average, as determined by the Independent Investment Banker, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Independent Investment Banker by the Reference Treasury Dealers at 3:30 p.m. New York time on the third business day preceding such redemption date.

“Treasury Rate” means, with respect to any redemption date, the rate per annum equal to the semiannual equivalent yield to maturity or interpolated (on a day count basis) of the Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such redemption date.

Notice of any redemption will be mailed or sent electronically pursuant to applicable DTC procedures at least 30 days but not more than 60 days before the redemption date to each holder of senior debentures to be redeemed. If the senior debentures to be redeemed are not global securities then held by DTC, the senior debentures to be redeemed will be selected by the trustee pursuant to the indenture.

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Unless we default in payment of the redemption price, on and after the redemption date interest will cease to accrue on the senior debentures or portions thereof called for redemption.

The Trustee

The Bank of New York Mellon Trust Company, N.A., will act as trustee, registrar, transfer agent and paying agent for the senior debentures. We can appoint a successor trustee so long as no event which is, or after notice or lapse of time would become, an event of default shall have occurred and be continuing.

We and certain of our affiliates maintain banking and other business relationships in the ordinary course of business with the trustee and its affiliates. In addition, the trustee and certain of its affiliates may serve as trustee for other securities issued by us or by our affiliates.

To the extent provided in the indenture, the trustee will have a prior claim on amounts held by it under the indenture for the payment of its compensation and expenses and for the repayment of advances made by it to effect performance of some covenants in the indenture.

Book-Entry Delivery and Settlement

We will issue the senior debentures in the form of one or more global certificates, which we refer to as global securities. We will deposit the global securities with or on behalf of DTC, and registered in the name of Cede & Co., as nominee of DTC, or else the global securities will remain in the custody of the trustee in accordance with the FAST Balance Certificate Agreement between DTC and the trustee.

DTC has advised us that:

We have provided the following descriptions of the operations and procedures of DTC solely as a matter of convenience. These operations and procedures are solely within the control of DTC and are subject to change by them from time to time. Neither we, the underwriters nor the trustee take any responsibility for these operations or procedures, and you are urged to contact DTC or its participants directly to discuss these matters.

We expect that under procedures established by DTC:

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The laws of some jurisdictions require that purchasers of securities take physical delivery of those securities in the form of a certificate. For that reason, it may not be possible to transfer interests in a global security to those persons. In addition, because DTC can act only on behalf of its participants, who in turn act on behalf of persons who hold interests through participants, the ability of a person having an interest in a global security to pledge or transfer that interest to persons or entities that do not participate in DTC’s system, or otherwise to take actions in respect of that interest, may be affected by the lack of a physical definitive security in respect of that interest.

So long as DTC or its nominee is the registered owner of a global security, DTC or that nominee will be considered the sole owner or holder of the senior debentures represented by that global security for all purposes under the indenture and under the senior debentures. Except as described below, owners of beneficial interests in a global security will not be entitled to have senior debentures represented by that global security registered in their names, will not receive or be entitled to receive the senior debentures in the form of a physical certificate and will not be considered the owners or holders of the senior debentures under the indenture or under the senior debentures, and may not be entitled to give the trustee directions, instructions or approvals. Any notices required to be given to the holders while the senior debentures are global securities will be given to DTC. For that reason, each holder owning a beneficial interest in a global security must rely on DTC’s procedures and, if that holder is not a direct or indirect participant in DTC, on the procedures of the DTC participant through which that holder owns its interest, to exercise any rights of a holder of senior debentures under the indenture or the global security.

Neither we nor the trustee will have any responsibility or liability for any act or omission by DTC, including any aspect of DTC’s records relating to the senior debentures or relating to payments made by DTC on account of the senior debentures, or any responsibility to maintain, supervise or review any of DTC’s records relating to the senior debentures.

We will make payments on the senior debentures represented by the global securities to DTC or its nominee, as the registered owner of the senior debentures. We expect that when DTC or its nominee receives any payment on the senior debentures represented by a global security, DTC will credit participants’ accounts with payments in amounts proportionate to their beneficial interests in the global security as shown in DTC’s records. We also expect that payments by DTC’s participants to owners of beneficial interests in the global security held through those participants will be governed by standing instructions and customary practice as is now the case with securities held for the accounts of customers registered in the names of nominees for such customers. DTC’s participants will be responsible for those payments.

Payments on the senior debentures represented by the global securities will be made in immediately available funds. Transfers between participants in DTC will be made in accordance with DTC’s rules and will be settled in immediately available funds.

Certificated Senior Debentures

We will issue certificated senior debentures to each person that DTC identifies as the beneficial owner of senior debentures represented by the global securities upon surrender by DTC of the global securities only if:

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Neither we nor the trustee will be liable for any delay by DTC, its nominee or any direct or indirect participant in identifying the beneficial owners of the related senior debentures. We and the trustee may conclusively rely on, and will be protected in relying on, instructions from DTC or its nominee, including instructions about the registration and delivery, and the respective principal amounts, of the senior debentures to be issued.

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UNDERWRITING

Goldman Sachs & Co. LLC, MUFG Securities Americas Inc. and Wells Fargo Securities, LLC are acting as representatives of each of the underwriters named below. Subject to the terms and conditions set forth in an underwriting agreement among us and the representatives of the underwriters, we have agreed to sell to the underwriters, and each of the underwriters severally and not jointly has agreed to purchase from us, the principal amount of senior debentures set forth opposite its name below.

The underwriters have agreed, subject to the terms and conditions contained in the underwriting agreement, to purchase all of the senior debentures sold under the underwriting agreement if any of these senior debentures are purchased. If an underwriter defaults, the underwriting agreement provides that the purchase commitments of the non-defaulting underwriters may be increased or the underwriting agreement may be terminated.

The underwriters are offering the senior debentures, subject to prior sale, when, as and if issued to and accepted by them, subject to approval of legal matters by their counsel, including the validity of the senior debentures, and other conditions contained in the underwriting agreement, such as the receipt by the underwriters of officer’s certificates and legal opinions. The underwriters reserve the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part. The offering of the senior debentures by the underwriters is subject to receipt and acceptance and subject to the underwriters’ right to reject any order in whole or in part.

The senior debentures are a new issue of securities with no established trading market. The senior debentures will not be listed on any securities exchange or on any automated dealer quotation system. The underwriters may make a market in the senior debentures after completion of the offering, but will not be obligated to do so and may discontinue any market-making activities at any time without notice. No assurance can be given as to the liquidity of the trading market for the senior debentures or that an active public market for the senior debentures will develop. If an active public trading market for the senior debentures does not develop, the market price and liquidity of the senior debentures may be adversely affected.

Settlement

We expect that delivery of the senior debentures will be made to investors on or about                 , 2018, which will be the fifth business day following the date of this prospectus supplement (such settlement being referred to as “T+5”). Under Rule 15c6-1 of the Securities Exchange Act of 1934, trades in the secondary market generally are required to settle in two business days, unless the parties to a trade expressly agree otherwise. Accordingly, purchasers who wish to trade the senior debentures on any date prior to the second business day before delivery will be required by virtue of the fact that the senior debentures initially will settle in five business days to specify alternative settlement arrangements to prevent a failed settlement. Purchasers who wish to trade the senior debentures on any date prior to the second business day before delivery should consult their advisors.

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Commissions and Discounts

The representatives have advised us that the underwriters propose initially to offer the senior debentures to the public at the public offering price on the cover page of this prospectus supplement, and may offer the senior debentures to certain dealers at that price less a concession not in excess of     % of the principal amount of the senior debentures. The underwriters may allow, and the dealers may reallow, a discount not in excess of     % of the principal amount of the senior debentures to other dealers. After the initial public offering, the public offering price, concession and discount may be changed.

The expenses of this offering, not including the underwriting discount, are estimated at $            and are payable by us.

No Sale of Similar Securities

We have agreed, with exceptions, not to sell or transfer any of our senior debentures from the date of this prospectus supplement until the issuance of the senior debentures without first obtaining the written consent of the representatives on behalf of the underwriters. Specifically, we have agreed not to directly or indirectly:

This lockup provision applies to any of our debt securities and to securities convertible into or exchangeable or exercisable for our debt securities.

Indemnification

We have agreed to indemnify the several underwriters against certain liabilities, including certain liabilities under the Securities Act of 1933, as amended, or to contribute to payments the underwriters may be required to make in respect of those liabilities.

Price Stabilization and Short Position

In connection with the offering, the underwriters are permitted to engage in transactions that stabilize the market price of the senior debentures. Such transactions consist of bids or purchases to peg, fix or maintain the price of the senior debentures.

If the underwriters create a short position in the senior debentures in connection with the offering, i.e., if they sell more senior debentures than are listed on the cover page of this prospectus supplement, then the representatives may reduce that short position by purchasing senior debentures in the open market. Purchases of our senior debentures to stabilize or reduce a short position could cause the price of our senior debentures to be higher than it might be in the absence of such purchases.

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Neither we nor any of the underwriters makes any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of the senior debentures. In addition, neither we nor any of the underwriters makes any representation that the representatives will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

Other Relationships

The underwriters and their respective affiliates are full service financial institutions engaged in various activities, which may include sales and trading, commercial and investment banking, advisory, investment management, investment research, principal investment, hedging, market making, brokerage and other financial and non-financial activities and services. Certain of the underwriters and their respective affiliates have provided, and may in the future provide, a variety of these services to the issuer and to persons and entities with relationships with the issuer, for which they received or will receive customary fees and expenses. In particular, affiliates of each of Goldman Sachs & Co. LLC, MUFG Securities Americas Inc., Wells Fargo Securities, LLC, Comerica Securities, Inc., KeyBanc Capital Markets Inc. and U.S. Bancorp Investments, Inc. are lenders under our credit facility.

In the ordinary course of their various business activities, the underwriters and their respective affiliates, officers, directors and employees may purchase, sell or hold a broad array of investments and actively trade securities, derivatives, loans, commodities, currencies, credit default swaps and other financial instruments for their own account and for the accounts of their customers, and such investment and trading activities may involve or relate to assets, securities and/or instruments of ours (directly, as collateral securing other obligations or otherwise) and/or persons and entities with relationships with us. If any of the underwriters or their affiliates have a lending relationship with us, certain of those underwriters or their affiliates routinely hedge, and certain other of those underwriters may hedge, their credit exposure to us consistent with their customary risk management policies. Typically, these underwriters and their affiliates would hedge such exposure by entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities, including potentially the senior debentures offered hereby. Any such credit default swaps or short positions could adversely affect future trading prices of the senior debentures offered hereby. The underwriters and their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such assets, securities or instruments and may at any time hold, or recommend to clients that they should acquire, long and/or short positions in such assets, securities and instruments.

Selling Restrictions

European Economic Area

The senior debentures are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the European Economic Area, or EEA. For these purposes, a retail investor means a person who is one (or more) of: (i) a retail client as defined in point (11) of Article 4(1) of Directive 2014/65/EU, as amended, or the MiFID II; or (ii) a customer within the meaning of Directive 2002/92/EC, as amended, where that customer would not qualify as a professional client as defined in point (10) of Article 4(1) of MiFID II; or (iii) not a qualified investor as defined in Directive 2003/71/EC, as amended. Consequently no key information document required by Regulation (EU) No 1286/2014, as amended, or the PRIIPs Regulation, for offering or selling the senior debentures or otherwise making them available to retail investors in the EEA has been prepared and therefore offering or selling the senior debentures or otherwise making them available to any retail investor in the EEA may be unlawful under the PRIIPS Regulation.

United Kingdom

Each underwriter has represented and agreed that:

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Hong Kong

The senior debentures may not be offered or sold in Hong Kong by means of any document other than (i) in circumstances which do not constitute an offer to the public within the meaning of the Companies (Winding Up and Miscellaneous Provisions) Ordinance (Cap. 32 of the Laws of Hong Kong), or the Companies (Winding Up and Miscellaneous Provisions) Ordinance, or which do not constitute an invitation to the public within the meaning of the Securities and Futures Ordinance (Cap. 571 of the Laws of Hong Kong), or the Securities and Futures Ordinance, or (ii) to “professional investors” as defined in the Securities and Futures Ordinance and any rules made thereunder, or (iii) in other circumstances which do not result in the document being a “prospectus” as defined in the Companies (Winding Up and Miscellaneous Provisions) Ordinance, and no advertisement, invitation or document relating to the senior debentures may be issued or may be in the possession of any person for the purpose of issue (in each case whether in Hong Kong or elsewhere), which is directed at, or the contents of which are likely to be accessed or read by, the public in Hong Kong (except if permitted to do so under the securities laws of Hong Kong) other than with respect to senior debentures which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” in Hong Kong as defined in the Securities and Futures Ordinance and any rules made thereunder.

Singapore

This prospectus supplement has not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus supplement and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the senior debentures may not be circulated or distributed, nor may the senior debentures be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor (as defined in Section 4A of the Securities and Futures Act, Chapter 289 of Singapore, or the SFA)) under Section 274 of the SFA, (ii) to a relevant person (as defined in Section 275(2) of the SFA ) pursuant to Section 275(1) of the SFA, or any person pursuant to Section 275(1A) of the SFA, and in accordance with the conditions specified in Section 275 of the SFA or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA, in each case subject to conditions set forth in the SFA.

Where the senior debentures are subscribed or purchased under Section 275 of the SFA by a relevant person which is a corporation (which is not an accredited investor (as defined in Section 4A of the SFA)) the sole business of which is to hold investments and the entire share capital of which is owned by one or more individuals, each of whom is an accredited investor, the securities (as defined in Section 239(1) of the SFA) of that corporation shall not be transferable for 6 months after that corporation has acquired the senior debentures under Section 275 of the SFA except: (1) to an institutional investor under Section 274 of the SFA or to a relevant person (as defined in Section 275(2) of the SFA), (2) where such transfer arises from an offer in that corporation’s securities pursuant to Section 275(1A) of the SFA, (3) where no consideration is or will be given for the transfer, (4) where the transfer is by operation of law, (5) as specified in Section 276(7) of the SFA, or (6) as specified in Regulation 32 of the Securities and Futures (Offers of Investments) (Shares and Debentures) Regulations 2005 of Singapore, or the Regulation 32).

Where the senior debentures are subscribed or purchased under Section 275 of the SFA by a relevant person which is a trust (where the trustee is not an accredited investor (as defined in Section 4A of the SFA)) whose sole purpose is to hold investments and each beneficiary of the trust is an accredited investor, the beneficiaries’ rights

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and interest (howsoever described) in that trust shall not be transferable for 6 months after that trust has acquired the senior debentures under Section 275 of the SFA except: (1) to an institutional investor under Section 274 of the SFA or to a relevant person (as defined in Section 275(2) of the SFA), (2) where such transfer arises from an offer that is made on terms that such rights or interest are acquired at a consideration of not less than S$200,000 (or its equivalent in a foreign currency) for each transaction (whether such amount is to be paid for in cash or by exchange of securities or other assets), (3) where no consideration is or will be given for the transfer, (4) where the transfer is by operation of law, (5) as specified in Section 276(7) of the SFA, or (6) as specified in Regulation 32.

Japan

The senior debentures have not been and will not be registered under the Financial Instruments and Exchange Act of Japan (Act No. 25 of 1948, as amended), or the FIEA. The senior debentures may not be offered or sold, directly or indirectly, in Japan or to or for the benefit of any resident of Japan (including any person resident in Japan or any corporation or other entity organized under the laws of Japan) or to others for reoffering or resale, directly or indirectly, in Japan or to or for the benefit of any resident of Japan, except pursuant to an exemption from the registration requirements of the FIEA and otherwise in compliance with any relevant laws and regulations of Japan.

Switzerland

This prospectus supplement does not constitute an issue prospectus pursuant to Article 652a or Article 1156 of the Swiss Code of Obligations and the senior debentures will not be listed on the SIX Swiss Exchange. Therefore, this prospectus supplement may not comply with the disclosure standards of the listing rules (including any additional listing rules or prospectus schemes) of the SIX Swiss Exchange. Accordingly, the senior debentures may not be offered to the public in or from Switzerland, but only to a selected and limited circle of investors who do not subscribe to the senior debentures with a view to distribution. Any such investors will be individually approached by the underwriters from time to time.

Canada

The senior debentures may be sold in Canada only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions or subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the senior debentures must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 of National Instrument 33-105 Underwriting Conflicts (NI 33-105), the underwriters are not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

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LEGAL MATTERS

The validity of the senior debentures will be passed upon for us by Perkins Coie LLP. Certain legal matters relating to Iowa law will be passed upon for us by Simmons Perrine Moyer Bergman PLC. The underwriters have been represented by Gibson, Dunn & Crutcher LLP.

EXPERTS

The consolidated financial statements, and the related financial statement schedule, incorporated in this prospectus supplement and the accompanying prospectus by reference from Interstate Power and Light Company’s Annual Report on Form 10-K for the year ended December 31, 2017 have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their report, which is incorporated herein by reference. Such consolidated financial statements and financial statement schedule have been so incorporated in reliance upon the report of such firm given upon their authority as experts in accounting and auditing.

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PROSPECTUS

Interstate Power and Light Company

Preferred Stock

Debt Securities

We may offer and sell from time to time, in one or more issuances in amounts, at prices and on terms determined at the time of offering, the following securities:

This prospectus provides you with a general description of the securities we may offer. Each time securities are sold using this prospectus, we will provide one or more prospectus supplements containing specific information about the offering and the terms of the securities being sold, including the offering price. Each prospectus supplement may also add, update or change information contained in this prospectus. You should carefully read this prospectus and the prospectus supplement relating to the specific issue of securities, together with the documents we incorporate by reference, before you invest.

We may offer and sell these securities to or through underwriters, dealers or agents, or directly to investors, on a continuous or a delayed basis. The applicable prospectus supplement will provide the specific terms of the plan of distribution.

Investing in our securities involves risk. See “Risk Factors” on page 4 of this prospectus and in any prospectus supplement or in such other document we refer you to in any prospectus supplement for a discussion of certain risks that prospective investors should consider before investing in our securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

This prospectus is dated December 15, 2017.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS

References in this prospectus to “we,” “us” and “our” refer to Interstate Power and Light Company, unless the context otherwise requires.

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission, or the SEC, utilizing a “shelf” registration process. Under this shelf registration process, we may, from time to time, sell the securities or combinations of the securities described in this prospectus in one or more offerings. This prospectus provides you with a general description of those securities. Each time we offer securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering. The prospectus supplement may also add, update or change information contained in this prospectus and may include other special considerations applicable to such offering of securities. If there is any inconsistency between the information in this prospectus and any prospectus supplement, you should rely on the information in the prospectus supplement. You should read this prospectus and any prospectus supplement together with the additional information described under the heading “Where You Can Find More Information.”

You should rely only on the information contained or incorporated by reference in this prospectus and in any prospectus supplement. “Incorporated by reference” means that we can disclose important information to you by referring you to another document filed separately with the SEC. We have not authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it.

This prospectus and any prospectus supplement or information incorporated by reference herein or therein contains summaries of certain agreements that we have filed as exhibits to various SEC filings, as well as certain agreements that we will enter into in connection with the offering of securities covered by any prospectus supplement. The descriptions of these agreements contained in this prospectus and any prospectus supplement or information incorporated by reference herein or therein do not purport to be complete and are subject to, or qualified in their entirety by reference to, the definitive agreements. Copies of the definitive agreements will be made available without charge to you by making a written or oral request to us. See “Where You Can Find More Information.”

We are not making offers to sell nor soliciting offers to buy, nor will we make an offer to sell nor solicit an offer to buy, securities in any jurisdiction where the offer or sale is not permitted.

You should assume that the information appearing in this prospectus and any supplement to this prospectus, or the information we file or previously filed with the SEC that we incorporate by reference in this prospectus or any prospectus supplement, is accurate only as of their respective dates. Our business, financial condition, results of operations and prospects may have changed since those dates.

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FORWARD-LOOKING STATEMENTS

This prospectus, any prospectus supplement and the information incorporated by reference herein and therein contain forward-looking statements intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements, other than statements of historical fact, included in this prospectus or any prospectus supplement or incorporated by reference herein or therein, including statements regarding anticipated financial performance, business strategy and management’s plans and objectives for future operations, are forward-looking statements. These forward-looking statements can be identified as such because the statements generally include words such as “may,” “believe,” “expect,” “anticipate,” “plan,” “project,” “will,” “projections,” “estimate,” or other words of similar import. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from those expressed in, or implied by, these statements. Readers are cautioned not to place undue reliance on these forward-looking statements. All forward-looking statements included in this prospectus, any prospectus supplement or in a document incorporated by reference herein or therein speak only as of the date of this prospectus, the applicable prospectus supplement or the document incorporated by reference, as the case may be. Additional information concerning factors that could cause actual results to differ materially from those in the forward-looking statements is contained under “Risk Factors” on page 4 of this prospectus and other documents that we file from time to time with the SEC that are incorporated by reference into this prospectus and any prospectus supplement. Numerous important factors described in this prospectus, any prospectus supplement and/or any document incorporated by reference in this prospectus and/or any prospectus supplement could affect these statements and could cause actual results to differ materially from our expectations. We assume no obligation, and disclaim any duty, to update or revise publicly any forward-looking statements, whether as a result of new information, future events or otherwise, except as required by law.

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INTERSTATE POWER AND LIGHT COMPANY

We are a public utility primarily serving retail customers in Iowa. We are engaged principally in:

All of our common stock is owned by Alliant Energy Corporation, a regulated investor-owned public utility holding company with subsidiaries, including us, serving primarily electricity and natural gas customers in the Midwest.

We are subject to the jurisdiction of the Iowa Utilities Board. We are also subject to the jurisdiction of the Federal Energy Regulatory Commission. Our parent corporation, Alliant Energy Corporation, is a “holding company” and we are a “subsidiary company” within the Alliant Energy Corporation “holding company system” as defined under the Public Utility Holding Company Act of 2005. As a result, we are subject to some of the regulatory provisions of such Act.

Our principal executive offices are located at Alliant Energy Tower, Cedar Rapids, Iowa 52401, and our telephone number is (319) 786-4411.

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RISK FACTORS

Investing in our securities involves risk. You should carefully consider the specific risks discussed or incorporated by reference in the applicable prospectus supplement, together with all the other information contained in the prospectus supplement or incorporated by reference in this prospectus. You should also consider the risks and uncertainties discussed under the caption “Risk Factors” included in our Annual Report on Form 10-K for the year ended December 31, 2016, which is incorporated by reference in this prospectus, and which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future.

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RATIO OF EARNINGS TO FIXED CHARGES AND RATIO OF EARNINGS TO COMBINED FIXED CHARGES AND PREFERRED DIVIDEND REQUIREMENTS

The following table shows our ratio of earnings to fixed charges and ratio of earnings to combined fixed charges and preferred dividend requirements for the periods presented:

USE OF PROCEEDS

Unless we inform you otherwise in the applicable prospectus supplement, we intend to use the net proceeds from the sale of the securities for general corporate purposes, which may include repayment or refinancing of debt, acquisitions, working capital, capital expenditures, investments and repurchases and redemptions of securities. Net proceeds may be temporarily invested prior to use.

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DESCRIPTION OF PREFERRED STOCK

The following description of our preferred stock summarizes certain general terms and provisions that apply to our preferred stock. The summary may not contain all of the information that is important to you and is subject to and qualified in its entirety by reference to our amended and restated articles of incorporation, which is filed as an exhibit to the registration statement of which this prospectus is a part. See “Where You Can Find More Information.”

We will describe the particular terms of any series of preferred stock more specifically in each prospectus supplement relating to that series of preferred stock. We will indicate in the prospectus supplement whether the general terms and provisions described in this prospectus apply to a particular series of preferred stock.

General

Our total authorized capital stock as set forth in our amended and restated articles of incorporation consists of 40,000,000 shares, of which 24,000,000 are designated common stock, par value $2.50 per share, and 16,000,000 shares are designated preferred stock, par value $0.01 per share. As of the date of this prospectus, all of our outstanding common stock was owned by our parent corporation, Alliant Energy Corporation and we had 8,000,000 shares of our 5.100% Series D cumulative perpetual preferred stock issued and outstanding.

Under our amended and restated articles of incorporation, our board of directors may establish one or more series of preferred stock to be issued out of authorized preferred stock. Our board of directors, without approval of our shareowners, may determine the rights and preferences of the shares of preferred stock of any series so established.

Terms

Prior to the issuance of shares of each series of our preferred stock, our board of directors is required to adopt resolutions and file articles of amendment to our amended and restated articles of incorporation with the Secretary of State of the State of Iowa. The articles of amendment will fix for each series the designation and number of shares and preferences, limitations, relative rights and other terms of the shares including, among other things:

In addition to the terms listed above, we will set forth in a prospectus supplement the following terms relating to the series of preferred stock being offered:

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All shares of our preferred stock will, when issued, be fully paid and nonassessable and will not have any preemptive or similar rights.

Ranking

The preferred stock will rank, with respect to dividends and upon our liquidation, dissolution or winding up:

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DESCRIPTION OF DEBT SECURITIES

The following description of our debt securities sets forth certain general terms and provisions of the debt securities to which any prospectus supplement may relate. The following description does not purport to be complete and is subject to, and is qualified in its entirety by reference to, the indenture between us and The Bank of New York Mellon Trust Company, N.A., as trustee, dated August 20, 2003, as it may be supplemented and amended from time to time. The indenture is filed as an exhibit to the registration statement of which this prospectus is a part and is incorporated by reference into this prospectus. See “Where You Can Find More Information.” The terms of the debt securities will include those stated in the indenture and those made a part of the indenture by reference to the Trust Indenture Act of 1939, as amended.

The particular terms of the debt securities offered by any prospectus supplement and the extent, if any, to which the provisions described in this prospectus may apply to the offered debt securities will be described in the prospectus supplement relating to the offered debt securities. Accordingly, for a description of the terms of a particular issue of debt securities, reference must be made to both the prospectus supplement relating thereto and to the following description.

General

The indenture does not limit the aggregate principal amount of senior unsecured debt securities that we may issue under it, and provides that we may issue securities under the indenture from time to time in one or more series pursuant to the terms of one or more supplemental indentures, board resolutions or officer’s certificates creating the series.

Terms

We will describe in each prospectus supplement the following terms that apply to the debt securities offered under that prospectus supplement:

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Ranking

The debt securities will be our senior, unsecured and unsubordinated obligations, ranking equally and ratably with all our other senior, unsecured and unsubordinated obligations. The debt securities will be effectively subordinated to all of our future secured indebtedness.

Payments

Unless we otherwise state in the prospectus supplement, we will pay principal of, and premium and interest on, if any, the debt securities at the office or agency we maintain for that purpose, initially the corporate trust office of the trustee. We will pay interest on debt securities issued in registered form on any interest payment date to the registered owners of the debt securities at the close of business on the regular record date for the interest payment date. We will name in the prospectus supplement all paying agents we initially designate for the debt securities. We may designate additional paying agents, rescind the designation of any paying agent or approve a change in the office through which any paying agent acts, but we must maintain a paying agent in each place where payments on the debt securities are payable.

Purchase and Cancellation

We may at any time purchase debt securities in the open market or otherwise at any price, subject to applicable U.S. securities laws. Any debt securities so purchased must be promptly surrendered to the trustee for cancellation.

Material U.S. Federal Income Tax Considerations

We may issue the debt securities as original issue discount securities, bearing no interest or bearing interest at a rate, which, at the time of issuance, is below market rates, to be sold at a substantial discount below their principal amount. We will describe some material U.S. federal income tax and other considerations applicable to any debt securities that are issued as original issue discount securities in the applicable prospectus supplement.

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If the purchase price of any debt securities is payable in one or more foreign currencies or composite currencies, if any debt securities are denominated in one or more foreign currencies or composite currencies or if any payments on the debt securities are payable in one or more foreign currencies or composite currencies, then we will describe the restrictions, elections, some U.S. federal income tax considerations, specific terms and other information about the debt securities and the foreign currency or composite currencies in the prospectus supplement.

Restrictive Covenants

Except as otherwise set forth under “—Satisfaction and Discharge” below, for so long as any debt securities remain outstanding or any amount remains unpaid on any of the debt securities, we will comply with the terms of the covenants set forth below. If we issue additional series of securities under the indenture in the future, those series may or may not have different covenants.

Limitations on Liens

The indenture provides that we will not, and we will not permit any of our subsidiaries to, create or allow to be created or to exist any lien on any of our properties or assets to secure any indebtedness, without making effective provision that makes the debt securities to which this limitation applies equally and ratably secured with or prior to all such indebtedness and with any other indebtedness that is also entitled to be equally secured. This restriction does not apply to or prevent the creation or existence of:

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This restriction will not apply to or prevent the creation or existence of leases we enter into, or on existing property we acquire, in the ordinary course of our business.

Consolidation, Merger and Sale of Assets

The indenture provides that we will not consolidate with or merge into any other corporation, or sell all or substantially all of our assets to any other person unless:

The indenture further provides that our successor will be substituted for us, after which all of our obligations under the indenture will terminate.

Events of Default

Each of the following will be an event of default with respect to the debt securities of a series under the indenture:

If an event of default occurs and continues, then either the trustee or the holders of at least 33% in principal amount of the outstanding debt securities of the series in default may declare the principal amount of the debt securities immediately due and payable.

The trustee can require that it be indemnified before it enforces the indenture or any outstanding debt securities. Subject to some limitations, holders of a majority in principal amount of the outstanding debt

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securities of a series can direct the trustee in its exercise of any trust or power. The trustee does not have to give holders notice of any continuing default, except a default in payment of principal or interest, if it in good faith determines that withholding notice is in the interests of the holders. We are required to give the trustee a certificate certifying as to our compliance with all conditions and covenants under the indenture at least once a year.

Modification of the Indenture

The holders of at least a majority in principal amount of outstanding debt securities of a series may waive any existing default and its consequences under the indenture. However, holders cannot waive (i) a default in the payment of the principal of, or premium, if any, or interest on, any debt securities or (ii) a default in respect of a provision that cannot be modified or amended without the consent of each holder of the outstanding debt securities of the series.

With the consent of the holders of at least a majority in aggregate principal amount of outstanding debt securities, we and the trustee can enter into supplemental indentures to amend or modify the indenture. If a proposed supplemental indenture affects the rights of the holders of one or more, but less than all, of the outstanding series of debt securities, or tranches thereof, then we and the trustee can enter into such supplemental indenture with the consent only of the holders of at least a majority in aggregate principal amount of outstanding series, or tranches thereof, so affected, voting together as one class. However, we cannot make modifications or amendments without the consent of all of the holders of the outstanding series of debt securities if the amendments or modifications would:

We and the trustee can also enter into supplemental indentures to amend or modify the indenture or the debt securities without the consent of any holders of the debt securities. We can only do so if those amendments or modifications would be limited to specific purposes, including:

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Satisfaction and Discharge

The indenture provides that we can at any time terminate almost all of our obligations with respect to any outstanding debt securities and the indenture. We cannot, however terminate some obligations, including our obligations to register the transfer or exchange of the debt securities, replace mutilated, destroyed, lost or stolen debt securities, to maintain agencies in respect of the debt securities and hold moneys for payment in trust.

If we desire to exercise our option to satisfy and discharge our obligations under the indenture, then we must deposit in trust with the trustee or any paying agent (other than us) money or U.S. government obligations sufficient to pay the outstanding principal amount of the debt securities as well as the interest and premium, if any, on the debt securities to maturity. In addition, we must provide to the trustee and the paying agent:

Governing Law

The indenture and the debt securities will be governed by, and construed in accordance with, the laws of the State of New York.

Global Securities

We may issue the securities in whole or in part in the form of one or more global certificates or notes, which we refer to as global securities, that we will deposit with a depository or its nominee that we identify in the applicable prospectus supplement.

We will describe the specific terms of the depository arrangement covering the securities in the prospectus supplement relating to that series. We anticipate that the following provisions will apply to all depository arrangements.

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Upon the issuance of the securities in the form of one or more global securities, the depository or its custodian will credit, on its book-entry registration and transfer system, the number of shares or principal amount of securities of the individual beneficial interests represented by these global securities to the respective accounts of persons who have accounts with the depository. Ownership of beneficial interests in the global securities will be shown on, and the transfer of this ownership will be effected only through, records maintained by the depository or its nominee with respect to interests of participants and the records of participants with respect to interests of persons other than participants. These accounts initially will be designated by or on behalf of the underwriters, initial purchasers or agents, or by us if we offer and sell the securities directly, and ownership of beneficial interests in the global securities will be limited to participants or persons who hold interests through participants. Qualified institutional buyers may hold their interests in the global securities directly through the depository if they are participants in this system, or indirectly through organizations which are participants in this system. The laws of some states of the United States may require that some purchasers of securities take physical delivery of the securities in definitive registered form. These limits and the laws may impair your ability to own, transfer or pledge interests in the global securities.

So long as the depository, or its nominee, is the registered owner or holder of the securities, the depository or its nominee, as the case may be, will be considered the sole owner or holder of the securities represented by the global securities for all purposes. No beneficial owner of an interest in the global securities will be able to transfer that interest except in accordance with the depository’s procedures.

We will make dividend payments on, or payments of the principal of, and premium, if any, and interest on, the global securities to the depository or its nominee, as the case may be, as the registered owner of the global securities. We will not have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in the global securities or for maintaining, supervising or reviewing any records relating to the beneficial ownership interest.

We expect that the depository or its nominee, upon receipt of any dividend payment on, or payment of the principal of, and premium, if any, and interest on, the global securities, will credit participants’ accounts with payments in amounts proportionate to their respective beneficial interests in the securities as shown on the records of the depository or its nominee. We also expect that payments by participants to owners of beneficial interests in the global securities held through the participants will be governed by standing instructions and customary practice, as is now the case with securities held for the accounts of customers registered in the names of nominees for their customers. These payments will be the responsibility of the participants. Transfers between participants in the depository will be effected in the ordinary way through the depository’s settlement system in accordance with the depository rules and will be settled in same day funds.

We will issue securities in certificated form in exchange for global securities if:

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PLAN OF DISTRIBUTION

We may sell our securities in any one or more of the following ways from time to time: (1) through agents; (2) to or through underwriters; (3) through brokers or dealers; (4) directly by us to purchasers, including through a specific bidding, auction or other process; or (5) through a combination of any of these methods of sale. The applicable prospectus supplement will contain the terms of the transaction, name or names of any underwriters, dealers, agents and the respective amounts of securities underwritten or purchased by them, the initial public offering price of the securities, and the applicable agent’s commission, dealer’s purchase price or underwriter’s discount. Any dealers and agents participating in the distribution of the securities may be deemed to be underwriters, and compensation received by them on resale of the securities may be deemed to be underwriting discounts.

Any initial offering price, dealer purchase price, discount or commission may be changed from time to time.

The securities may be distributed from time to time in one or more transactions, at negotiated prices, at a fixed price or fixed prices (that may be subject to change), at market prices prevailing at the time of sale, at various prices determined at the time of sale or at prices related to prevailing market prices.

Offers to purchase securities may be solicited directly by us or by agents designated by us from time to time. Unless otherwise indicated in the prospectus supplement, any such agent will use its commercially reasonable efforts to solicit purchases for the period of its appointment or to sell securities on a continuing basis. Agents may receive compensation in the form of commissions, discounts or concessions from us. Agents may also receive compensation from the purchasers of the securities for whom they sell as principals. Each particular agent will receive compensation in amounts negotiated in connection with the sale, which might be in excess of customary commissions. Any such agent may be deemed to be an underwriter, as that term is defined in the Securities Act of 1933, or the Securities Act, of the securities so offered and sold. Accordingly, any commission, discount or concession received by them and any profit on the resale of the securities purchased by them may be deemed to be underwriting discounts or commissions under the Securities Act. We have not entered into any agreements, understandings or arrangements with any underwriters or broker-dealers regarding the sale of their securities. As of the date of this prospectus, there are no special selling arrangements between any broker-dealer or other person and us. No period of time has been fixed within which the securities will be offered and sold.

If underwriters are utilized in the sale of any securities in respect of which this prospectus is being delivered, such securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including negotiated transactions, at fixed public offering prices or at varying prices determined by the underwriters at the time of sale. Securities may be offered to the public either through underwriting syndicates represented by managing underwriters or directly by one or more underwriters. If any underwriter or underwriters are utilized in the sale of securities, unless otherwise indicated in the applicable prospectus supplement, the obligations of the underwriters are subject to certain conditions precedent, and the underwriters will be obligated to purchase all such securities if they purchase any of them.

If a dealer is utilized in the sale of the securities in respect of which this prospectus is delivered, we will sell such securities to the dealer as principal. The dealer may then resell such securities to the public at varying prices to be determined by such dealer at the time of resale. Transactions through brokers or dealers may include block trades in which brokers or dealers will attempt to sell shares as agent but may position and resell as principal to facilitate the transaction or in cross trades, in which the same broker or dealer acts as agent on both sides of the trade. Any such dealer may be deemed to be an underwriter, as such term is defined in the Securities Act, of the securities so offered and sold.

Offers to purchase securities may be solicited directly by us, and the sale thereof may be made by us, directly to institutional investors or others who may be deemed to be underwriters within the meaning of the Securities Act with respect to any resale thereof.

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Agents, underwriters and dealers may be entitled under relevant agreements with us to indemnification by us against certain liabilities, including liabilities under the Securities Act, or to contribution with respect to payments which such agents, underwriters and dealers may be required to make in respect thereof. The terms and conditions of any indemnification or contribution will be described in the applicable prospectus supplement.

We may also sell securities through various arrangements involving mandatorily or optionally exchangeable securities, and this prospectus may be delivered in connection with those sales.

We may enter into derivative, sale or forward sale transactions with third parties, or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If the applicable prospectus supplement indicates, in connection with those transactions, the third parties may sell securities covered by this prospectus and the applicable prospectus supplement, including in short sale transactions and by issuing securities not covered by this prospectus but convertible into, exchangeable for or representing beneficial interests in securities covered by this prospectus, or the return of which is derived in whole or in part from the value of such securities.

Underwriters, broker-dealers or agents may receive compensation in the form of commissions, discounts or concession. Underwriters, broker-dealers or agents may also receive compensation from the purchasers of securities for whom they act as agents or to whom they sell as principals, or both. Compensation as to a particular underwriter, broker-dealer or agent will be in amounts to be negotiated and might be in excess of customary commissions. In effecting sales, broker-dealers may arrange for other broker-dealers to participate in the resales.

Any securities offered other than common stock will be a new issue and will have no established trading market. We may elect to list any series of securities on an exchange, but, unless otherwise specified in the applicable prospectus supplement and/or other offering material, we shall not be obligated to do so. No assurance can be given as to the liquidity of the trading market for any of the securities.

Agents, underwriters and dealers may engage in transactions with, or perform services for, us or our subsidiaries in the ordinary course of business.

Any underwriter may engage in overallotment, stabilizing transactions, short covering transactions and penalty bids in accordance with Regulation M under the Securities Exchange Act of 1934. Overallotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time. An underwriter may carry out these transactions on an exchange, in the over-the-counter market or otherwise.

The place and time of delivery for securities will be set forth in the accompanying prospectus supplement.

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WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports and other information with the SEC (File No. 1-4117). We also filed a registration statement on Form S-3, including exhibits, under the Securities Act with respect to the securities offered by this prospectus. This prospectus is a part of the registration statement, but does not contain all of the information included in the registration statement or the exhibits to the registration statement. You may read and copy the registration statement and any other materials that we file with the SEC at the SEC’s Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549. You may call the SEC at 1-800-SEC-0330 for information on the operation of the Public Reference Room. Our reports and other SEC filings are also available at the SEC’s web site at http://www.sec.gov.

We are “incorporating by reference” specified documents that we file with the SEC, which means:

We incorporate by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, after the date of this prospectus and before the end of the offering of the securities pursuant to this prospectus:

Notwithstanding the foregoing, information furnished under Items 2.02 and 7.01 of any Current Report on Form 8-K, including the related exhibits under Item 9.01, is not incorporated by reference in this prospectus.

Some of these reports, however, are filed on a combined basis with our parent, Alliant Energy Corporation, and its direct subsidiary, Wisconsin Power and Light Company. Information contained in these reports relating to these entities is filed by them on their own behalf and not by us.

You may obtain copies of documents incorporated by reference in this prospectus, at no cost, by request directed to us at the following address or telephone number:

Interstate Power and Light Company

Alliant Energy Tower

Cedar Rapids, Iowa 52401

Attention: James H. Gallegos, Senior Vice

President, General Counsel and Corporate

Secretary

(319) 786-4411

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Any statement contained herein or in a document incorporated or deemed to be incorporated by reference herein will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained herein, in any other subsequently filed document which also is or is deemed to be incorporated by reference herein or in any prospectus supplement, modifies or supersedes such statement. Any such statement so modified or superseded will not be deemed, except as so modified and superseded, to constitute a part of this prospectus.

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LEGAL MATTERS

Unless otherwise specified in a prospectus supplement accompanying this prospectus, the validity of the securities offered in this prospectus and certain legal matters will be passed upon for us by Perkins Coie LLP, and, with respect to certain matters of Iowa law, Nyemaster Goode, P.C. If the securities are being distributed in an underwritten offering, certain legal matters will be passed upon for the underwriters by counsel identified in the related prospectus supplement.

EXPERTS

The consolidated financial statements, and the related financial statement schedule, incorporated in this prospectus by reference from Interstate Power and Light Company’s 2016 Annual Report on Form 10-K have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their report, which is incorporated herein by reference. Such financial statements and financial statement schedule have been so incorporated in reliance upon the report of such firm given upon their authority as experts in accounting and auditing.

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$            

Interstate Power and Light Company

    % Senior Debentures due 20    

PROSPECTUS SUPPLEMENT

Joint Book-Running Managers

Goldman Sachs & Co. LLC

MUFG

Wells Fargo Securities

Co-Managers

Comerica Securities

KeyBanc Capital Markets

US Bancorp

Mischler Financial Group, Inc.

Ramirez & Co., Inc.

                    , 2018