X0306 4 2018-12-17 0 0001609809 Seres Therapeutics, Inc. MCRB 0001643756 Aunins John G. C/O SERES THERAPEUTICS, INC. 200 SIDNEY STREET CAMBRIDGE MA 02139 0 1 0 0 Chief Technology Officer & EVP COMMON STOCK 2018-12-17 4 S 0 14282 6.3936 D 75250 D COMMON STOCK 2018-12-19 4 M 0 10000 A 85250 D RESTRICTED STOCK UNITS 2018-12-19 4 M 0 10000 0.00 D COMMON STOCK 10000 0 D The sale reported in the Form 4 was effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on March 15, 2018. The price reported is a weighted average price. These shares were sold in multiple transactions at per share prices ranging from $6.2100 to $6.5900. The Reporting Person undertakes to provide upon request to the SEC staff, the Issuer, or any stockholder of the Issuer, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. Each restricted stock unit represents right to receive one share of Issuer common stock. These restricted stock units have vested and settled as to 25% of the restricted stock units on December 19, 2017, 25% on June 19, 2018; and 50% on December 19, 2018. /s/ Thomas J. DesRosier, Attorney-in-Fact 2018-12-19