SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 10-K/A

For the fiscal year ended December 31, 1999

Commission file number 0-4804

TENNANT COMPANY
Incorporated in the State of Minnesota            Employer Identification Number 41-0572550

701 North Lilac Drive, P.O. Box 1452, Minneapolis, Minnesota 55440

Telephone Number 763-540-1208

Securities registered pursuant to Section 12 (b) of the Act: NONE

Securities registered pursuant to Section 12 (g) of the Act:

Common Stock, par value $.375 per share

and

Preferred Share Purchase Rights

    Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15 (d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes /x/  No / /

    Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant's knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K. /x/

    $283,601,511.50 is aggregate market value of common stock held by non-affiliates as of March 6, 2000.

9,039,092 shares outstanding at March 6, 2000

DOCUMENTS INCORPORATED BY REFERENCE

    2000 Proxy—Part III (Partial)

John T. Pain, Vice President, Treasurer and Chief Financial Officer

John T. Pain (51) joined the Company in 1984 as Corporate Tax Manager. He was named Assistant Treasurer in 1986, Corporate Controller and Principal Accounting Officer in 1997, and Vice President, Treasurer, and Chief Financial Officer in 1998. Mr. Pain is a Certified Public Accountant. He is a director of Castex Incorporated, and Tennant N.V.

Keith D. Payden, Vice President

Keith D. Payden (52) joined the Company in 1981. He was named Director, Information Services in 1987, Chief Information Officer in 1992, and Vice President in 1993.

James J. Seifert, Vice President, General Counsel and Secretary

James J. Seifert (43) joined the company in 1999 as Vice President, General Counsel and Secretary. Mr. Seifert is a director of Tennant UK Limited, Tennant Holding B.V., Tennant Europe B.V., Tennant, Import B.V., Tennant N.V., and Tennant Japan.

Thomas J. VanderBie, Senior Vice President

Thomas J. VanderBie (48) was named Senior Vice President of Tennant Company in August, 1999. Mr. VanderBie was President of Castex Incorporated, a subsidiary of Tennant Company from 1974-1999.

Steven K. Weeks, Vice President

Steven K. Weeks (44) joined the Company in 1984. He was named Manager, Global New Business and Marketing Development in 1993; Director of Marketing in 1994; Vice President, Customer Solutions in 1996; and Vice President, Global Marketing in 1999.

PART II

Item 5—Market for Registrant's Common Equity and Related Stockholder Matters

    The annual meeting of Tennant Company will be held at 10:30 a.m. on Thursday, May 4, at the Company's corporate headquarters, 701 North Lilac Drive, Golden Valley, Minnesota. The Company's common stock is traded in the National Market System of NASDAQ, under the ticker symbol TANT. As of December 31, 1999, there were approximately 3,500 shareholders of record. The accompanying chart shows the quarterly price range of the Company's shares over the past five years after adjustment for the two-for-one stock split:

    Cash dividends on the Company's common stock have been paid for 56 consecutive years, and the Company has increased dividends in each of the last 28 years. Dividends generally are declared each quarter. Following are the record dates anticipated for 2000: June 1, 2000; September 1, 2000; December 15, 2000.

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