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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , DC 20549 FORM 10 - K ( Mark One ) ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 Delaware ( State or Other Jurisdiction of Incorporation or Organization ) 200 Sidney Street - 4th Floor Cambridge , Massachusetts ( Address of Principal Executive Offices ) For the fiscal year ended December 31 , 2020 OR Title of each class Common stock , par value $ 0.001 per share For the transition period from to Seres Therapeutics , Inc. ( Exact Name of Registrant as Specified in Its Charter ) Commission File Number : 001-37465 □ □ □ ( 617 ) 945-9626 ( Registrant's Telephone Number , Including Area Code ) Securities registered pursuant to Section 12 ( b ) of the Act Trading Symbol ( s ) MCRB 27-4326290 ( IRS Employer Identification No. ) 02139 ( Zip Code ) Name of each exchange on which registered The Nasdaq Global Select Market Securities Registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . Yes 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 No No No Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S T ( § 232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes No Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of the " large accelerated filer , " " accelerated filer , " " smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Large accelerated filer Non - accelerated filer Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to section 13 ( a ) of the Exchange Act . Accelerated filer Smaller reporting company Indicate by check mark whether the registrant has filed a report on the attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . | Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Act ) . Yes No > The aggregate market value of the voting and non - voting common stock held by non - affiliates of the registrant based on the closing price of the registrant's common stock as reported on the Nasdaq Global Select Market on June 30 , 2020 , was $ 219,853,799 . Solely for purposes of this disclosure , shares of common stock held by executive officers , directors and certain stockholders of the registrant as of such date have been excluded because such holders may be deemed to be affiliates . As of February 24 , 2021 , there were 91,549,412 shares of the registrant's common stock , par value $ 0.001 per share , outstanding . DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant's definitive Proxy Statement relating to its 2021 Annual Meeting of Stockholders to be filed with the SEC within 120 days after the end of the fiscal year ended December 31 , 2020 are incorporated herein by reference in Part III .