FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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(Print or Type Responses)

1. Name and Address of Reporting Person * AFEYAN NOUBAR   2. Issuer Name and Ticker or Trading Symbol Seres Therapeutics, Inc. [MCRB] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director __X__ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 1 MEMORIAL DRIVE, 7TH FLOOR 3. Date of Earliest Transaction (Month/Day/Year) 06/25/2015 (Street) CAMBRIDGE, MA 02142 4. If Amendment, Date Original Filed(Month/Day/Year) 6. Individual or Joint/Group Filing(Check Applicable Line) ___ Form filed by One Reporting Person _X_ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock, $0.001 par value 06/26/2015   C   8,822,420 A (4) 8,822,420 I See Footnote (1) Common Stock, $0.001 par value 06/26/2015   C   2,205,603 A (4) 2,205,603 I See Footnote (2) Common Stock, $0.001 par value 06/26/2015   C   599,297 A (4) 599,297 I See Footnote (3) Common Stock, $0.001 par value               3,055,556 I See Footnote (6)

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Underlying Securities (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Series A Convertible Preferred Stock	(4)	06/26/2015		C			6,105,362	(4)	(4)	Common Stock	6,105,362	$ 0	0	I	See Footnote (1)
Series A Convertible Preferred Stock	(4)	06/26/2015		C			1,526,338	(4)	(4)	Common Stock	1,526,338	$ 0	0	I	See Footnote (2)
Series A Convertible Preferred Stock	(4)	06/26/2015		C			599,297	(4)	(4)	Common Stock	599,297	$ 0	0	I	See Footnote (3)
Series A-2 Convertible Preferred Stock	(4)	06/26/2015		C			898,877	(4)	(4)	Common Stock	898,877	$ 0	0	I	See Footnote (1)
Series A-2 Convertible Preferred Stock	(4)	06/26/2015		C			224,719	(4)	(4)	Common Stock	224,719	$ 0	0	I	See Footnote (2)
Series B Convertible Preferred Stock	(4)	06/26/2015		C			1,818,181	(4)	(4)	Common Stock	1,818,181	$ 0	0	I	See Footnote (1)
Series B Convertible Preferred Stock	(4)	06/26/2015		C			454,546	(4)	(4)	Common Stock	454,546	$ 0	0	I	See Footnote (2)
Stock Option (right to buy)	$ 18	06/25/2015		A		15,000		(5)	06/24/2025	Common Stock	15,000	$ 0	15,000	I	See Footnote (5)

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
AFEYAN NOUBAR 1 MEMORIAL DRIVE, 7TH FLOOR CAMBRIDGE, MA 02142	X	X
Flagship Ventures Fund 2007, L.P. 1 MEMORIAL DRIVE, 7TH FLOOR CAMBRIDGE, MA 02142		X
Flagship Ventures Fund IV, L.P. 1 MEMORIAL DRIVE, 7TH FLOOR CAMBRIDGE, MA 02142		X
Flagship Ventures Fund IV-Rx, L.P. 1 MEMORIAL DRIVE, 7TH FLOOR CAMBRIDGE, MA 02142		X
Flagship Ventures 2007 General Partner LLC 1 MEMORIAL DRIVE, 7TH FLOOR CAMBRIDGE, MA 02142		X
Flagship Ventures Fund IV General Partner LLC 1 MEMORIAL DRIVE, 7TH FLOOR CAMBRIDGE, MA 02142		X
KANIA EDWIN M JR 1 MEMORIAL DRIVE, 7TH FLOOR CAMBRIDGE, MA 02142		X

Signatures

FLAGSHIP VENTURES FUND 2007, L.P., By: Flagship Ventures 2007 General Partner, LLC, By: /s/ Noubar Afeyan, Name: Noubar B. Afeyan, Title: Manager		06/26/2015
**Signature of Reporting Person		Date
FLAGSHIP VENTURES 2007 GENERAL PARTNER, LLC, By: /s/ Noubar Afeyan, Name: Noubar B. Afeyan, Title: Manager		06/26/2015
**Signature of Reporting Person		Date
FLAGSHIP VENTURES FUND IV, L.P., By: Flagship Ventures Fund IV General Partner, LLC, By: /s/ Noubar Afeyan, Name: Noubar B. Afeyan, Title: Manager		06/26/2015
**Signature of Reporting Person		Date
FLAGSHIP VENTURES FUND IV GENERAL PARTNER LLC, By: /s/ Noubar Afeyan, Name: Noubar B. Afeyan, Title: Manager		06/26/2015
**Signature of Reporting Person		Date
FLAGSHIP VENTURES FUND IV-RX, L.P., By: Flagship Ventures 2007 General Partner, LLC, By: /s/ Noubar Afeyan, Name: Noubar B. Afeyan, Title: Manager		06/26/2015
**Signature of Reporting Person		Date
FLAGSHIP VENTURELABS IV, LLC, By: Flagship Ventures Fund IV General Partner LLC, By: /s/ Noubar Afeyan		06/26/2015
**Signature of Reporting Person		Date
NOUBAR B. AFEYAN, PH.D, /s/ Noubar Afeyan		06/26/2015
**Signature of Reporting Person		Date
EDWIN M. KANIA, JR., /s/ Edwin Kania, Jr.		06/26/2015
**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 4(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	Held by Flagship Ventures Fund IV, L.P. ("Flagship Fund IV"). Flagship Ventures Fund IV General Partner LLC ("Flagship Fund IV GP") is the general partner of Flagship Fund IV. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship Fund IV GP and each of these individuals may be deemed to share voting and investment power with respect to all shares held by Flagship Fund IV. Each of the filing persons disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein.
(2)	Held by Flagship Ventures Fund IV-Rx, L.P. ("Flagship IV-Rx"). Flagship Fund IV GP is the general partner of Flagship IV Rx. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship Fund IV GP and each of these individuals may be deemed to share voting and investment power with respect to all shares held by Flagship IV-Rx. Each of the filing persons disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein.
(3)	Held by Flagship Ventures Fund 2007, L.P. ("Flagship 2007"). Flagship Ventures 2007 General Partner LLC ("Flagship 2007 LLC") is the general partner of Flagship 2007. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship 2007 LLC and each of these individuals may be deemed to share voting and investment power with respect to all shares held by Flagship 2007. Each of the filing persons disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein.
(4)	Each class of preferred stock converted into Common Stock on a one-for-one basis upon the listing of the issuer's securities on The NASDAQ Global Select Exchange and had no expiration date.
(5)	Held by Noubar B. Afeyan, Ph.D. The option will vest as to 25% of the shares on June 25, 2016. The remainder of the shares will vest in 12 equal monthly installments.
(6)	Held by Flagship VentureLabs IV LLC ("Flagship VentureLabs"). Flagship Fund IV is a member of Flagship VentureLabs and also serves as its manager. Flagship Fund IV GP is the general partner of Flagship Fund IV. Noubar B. Afeyan, Ph.D. and Edwin M. Kania, Jr. are the managers of Flagship Fund IV GP. Each of these individuals and entities may be deemed to share voting and investment power with respect to all shares held by Flagship VentureLabs. Each of the filing persons disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein.

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