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As filed with the Securities and Exchange Commission on December 1, 2003

Registration No. 333-110564

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

Amendment No. 1

to

Form S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Infinity Property and Casualty Corporation

(Exact Name of Registrant as Specified in Its Charter)

2204 Lakeshore Drive

Birmingham, Alabama 35209

(205) 870-4000

(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices)

Samuel J. Simon, Esq.

Senior Vice President, General Counsel and Secretary

Infinity Property and Casualty Corporation

2204 Lakeshore Drive

Birmingham, Alabama 35209

Telephone: (205) 870-4000

Facsimile: (205) 803-8585

(Name, Address, Including Zip Code, Telephone and Facsimile Numbers, Including Area Code, of Agent For Service)

With copies to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after the effective date of this Registration Statement.

If any of the securities being registered on this form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.    ¨

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ¨

If delivery of the prospectus is expected to be made pursuant to Rule 434, please check the following box.    ¨

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, or until the Registration Statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

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

SUBJECT TO COMPLETION, DATED DECEMBER 1, 2003

PROSPECTUS

7,850,465 Common Shares

Infinity Property and Casualty Corporation

Common Stock

A wholly-owned subsidiary of American Financial Group, Inc. is selling all of the shares of common stock in this offering other than the shares subject to the over-allotment option described below. Infinity will not receive any of the proceeds from the sale of the shares by the selling shareholder. Our common stock is quoted on the Nasdaq National Market under the symbol “IPCC.” On November 25, 2003, the last quoted price of the shares of common stock as reported on the Nasdaq National Market was $34.00 per share.

Investing in our common stock involves risks that are described in the “ Risk Factors” section beginning on page 10 of this prospectus.

The underwriters may also purchase up to an additional 1,177,569 shares of common stock from us at the public offering price, less the underwriting discount, within 30 days from the date of this prospectus to cover over-allotments. If the underwriters exercise that option in full, the total public offering price, underwriting discount and proceeds to us would be $     , $     , and $     , respectively.

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.

The shares of common stock will be ready for delivery on or about                     , 2003.

Banc of America Securities LLC

Bear, Stearns & Co. Inc.

Morgan Keegan & Company, Inc.

The date of this prospectus is                    , 2003.

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You should rely only on the information contained in this document or to which we have referred you. We have not authorized anyone to provide you with information that is different. This document may only be used where it is legal to sell these securities. The information in this document may only be accurate on the date of this document.

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PROSPECTUS SUMMARY

This summary highlights information about Infinity Property and Casualty Corporation and the offering. Because this is a summary, it may not contain all the information you should consider before investing in our common stock. You should carefully read this entire prospectus.

References in this prospectus to “Infinity,” “we,” “us” and “our”, unless the context requires otherwise, refer to Infinity Property and Casualty Corporation and its combined operations, including the Assumed Agency Business. Unless otherwise noted in this prospectus, we assume that the underwriters will not exercise their over-allotment option. Unless otherwise indicated, all financial information in this prospectus is presented on a pro forma basis for periods prior to December 31, 2002 (see “Our Formation and Separation from AFG” and “Unaudited Pro Forma Financial Information”). In this prospectus, all share and per share data assumes that the stock split we declared in January 2003 had already occurred. Unless otherwise indicated, insurance industry data and our market share or ranking in the industry were derived from data compiled by A.M. Best Company Inc.

Infinity

What We Do

We are a national provider of personal automobile insurance with an emphasis on nonstandard auto insurance. Nonstandard auto insurance provides coverage to drivers who, due to their driving record, age or vehicle type, represent higher than normal risks and pay higher rates for comparable coverage. We also write standard and preferred personal auto insurance, nonstandard commercial auto insurance and complementary personal lines insurance products.

Our products are offered primarily through a network of approximately 14,000 independent agencies and strategic partnerships. Based on data published by A.M. Best, we believe we are the second largest provider of nonstandard auto coverage through independent agents in the United States, behind only The Progressive Corporation. While licensed to write insurance in every state, we focus on 15 states which we believe provide the greatest opportunity for profitable growth.

In 2002, we generated, on a pro forma basis, $988.9 million in gross premiums written, $687.3 million in net premiums written and had net earnings of $36.3 million. For the nine months ended September 30, 2003, we generated $734.8 million in gross premiums written, $622.8 million in net premiums written and had net earnings of $38.8 million. For the year ended December 31, 2002 and the nine months ended September 30, 2003, approximately 95% of our business was personal auto and the remaining 5% was homeowners, umbrella liability, boat owners and nonstandard commercial auto coverages. While there is no precise, industry-recognized definition of nonstandard auto insurance, we estimate that, for the year ended December 31, 2002 and the nine months ended September 30, 2003, approximately four-fifths of our personal auto business was nonstandard coverage. At September 30, 2003, we had total assets of $2.0 billion, total liabilities of $1.5 billion and shareholders’ equity of $442 million.

The following table compares our statutory combined ratio in past years with those of the personal lines insurance industry as a whole. See “Business — Our Strengths” for a description of how the combined ratio is calculated, the use of the combined ratio as a measure of underwriting profitability and the source of the industry combined ratios presented.

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Our Strengths

We believe that we are well positioned to compete in today’s market through various strengths that should enable us to build upon our history of favorable underwriting results. These strengths include:

Our Strategy

Our goal is to maximize shareholder value by focusing on underwriting profitability and long-term return on equity. We pursue this goal through a strategy of:

Certain Risks We Face

Our ability to capitalize on our strengths and implement our strategy entails risks. For example, we have a limited operating history as an independent public company. Further, the process of consolidating the operations of our insurance subsidiaries poses managerial, strategic and technological challenges. Adverse developments in the market for personal automobile insurance, or the personal automobile insurance industry in general, could cause our results of operations to suffer. Our reliance on the independent agency market makes us vulnerable to a reduction in the amount of business written by independent agents, and if we are not able to attract and retain independent agents, our revenues could be negatively affected. For further discussion of these and other risks we face, see “Risk Factors”.

Our principal executive offices are located at 2204 Lakeshore Drive, Birmingham, Alabama, 35209. Our telephone number is (205) 870-4000.

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The Offering

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Our Formation and Separation from AFG

Infinity was formed in September 2002 as an indirect wholly-owned subsidiary of American Financial Group, Inc. to acquire and conduct, as a separate public company, AFG’s personal lines insurance business written through independent agents. Prior to the initial public offering of common stock, AFG transferred to Infinity all of the issued and outstanding capital stock of the following personal auto insurance subsidiaries, including their respective subsidiaries, involved primarily in the issuance of nonstandard auto policies: Atlanta Casualty Company, Leader Insurance Company, Infinity Insurance Company and Windsor Insurance Company. We refer to these subsidiaries as the NSA Group. In this prospectus, we refer to our operations and financial results before January 1, 2003 as the operations and financial results of either Infinity or the NSA Group. In exchange for the NSA Group, AFG received all of the issued and outstanding shares of Infinity common stock and a note payable in the amount of $55 million.

In addition, as of January 1, 2003, AFG’s principal property and casualty insurance subsidiary, Great American Insurance Company (“Great American”), transferred to us its personal insurance business written through independent agents. This is primarily auto insurance for standard and preferred drivers, but also includes other personal lines. Because this business is not a separate legal entity, the transfer was effected through a reinsurance agreement under which we assumed the inforce business, service the policyholders and handle the claims. Great American, in turn, transferred to us assets (primarily investment securities) with a market value of $125.3 million, which was approximately equal to the net liabilities related to the inforce business, less

$5 million. We refer to this business as the Assumed Agency Business.

We completed the initial public offering of our common stock in February 2003. In the initial public offering, AFG sold 12.5 million shares of our common stock held by it. We received no proceeds from the initial public offering. AFG is selling its remaining shares in this offering.

The financial assets transferred to us from Great American in connection with our acquisition of Great American’s personal insurance business written through independent agents include primarily investment securities which are described in more detail in Note 1 to the Unaudited Pro Forma Financial Information. Other financial assets (primarily agents’ balances and deferred policy acquisition costs) and financial liabilities (primarily loss reserves and unearned premiums) transferred are described more fully in the Statement of Assets (excluding investments) and Liabilities to be Transferred for this business which begin on page F-18 of this Prospectus. In addition to the transfer of financial assets and liabilities, we received the operational processes, including policy renewal rights, and employees necessary to conduct the normal operations of this business following the date of transfer. Accordingly, the transfer represented the acquisition of a “business” under generally accepted accounting principles. Since the transfer was made while Infinity was an AFG subsidiary, the assets and liabilities transferred were recorded at AFG’s historical cost.

The companies and business comprising Infinity represented approximately 31% of AFG’s entire property and casualty group and approximately 83% of AFG’s Personal segment based on earned premiums in 2002. AFG’s property and casualty group was engaged primarily in specialty and private passenger automobile insurance businesses. The Specialty group includes a highly diversified group of specialty business units. Some of the more significant areas are inland and ocean marine, California workers’ compensation, agricultural-related coverages, executive and professional liability, fidelity and surety bonds, collateral protection, and umbrella and excess coverages. The Personal group wrote nonstandard and preferred/standard private passenger auto and other personal insurance coverage. AFG’s annuity and life business markets primarily retirement products as well as life and supplemental health insurance. AFG’s businesses operate throughout the United States. In 2002, 2001 and 2000, AFG derived less than 2% of its revenues from the sale of life and supplemental health products in Puerto Rico and less than 1% of its revenues from the sale of property and casualty insurance in Mexico, Canada, Puerto Rico, Europe and Asia.

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The following table (in millions) shows AFG’s revenues by significant business segment for the three years preceding our initial public offering of common stock.

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Summary Historical Financial Data

We derived the summary data as of and for each of the three years ended December 31, 2002, from financial statements audited by Ernst & Young LLP. We derived the summary data as of and for the nine months ended September 30, 2003 and 2002, from unaudited financial statements which include all adjustments, consisting of normal recurring accruals, that management considers necessary for a fair presentation of the financial position and results of operations for those periods. Results for the interim periods are not necessarily indicative of results to be expected for the entire year. The Infinity financial data set forth below for all periods prior to December 31, 2002 is the financial data of the NSA Group. The Infinity financial data for the nine months ended September 30, 2003 include the results of the Assumed Agency Business. You should read this summary in conjunction with “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the financial statements and related notes appearing elsewhere in this prospectus.