Initial Statement of Beneficial Ownership (3)

FORM 3	UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES	OMB APPROVAL OMB Number: 3235-0104 Estimated average burden hours per response... 0.5
	Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

1. Name and Address of Reporting Person ^*

Flagship Pioneering Inc.

2. Date of Event Requiring Statement (MM/DD/YYYY) 6/18/2019

3. Issuer Name and Ticker or Trading Symbol

Seres Therapeutics, Inc. [MCRB]

(Last) (First) (Middle)

55 CAMBRIDGE PARKWAY

4. Relationship of Reporting Person(s) to Issuer (Check all applicable)

_____ Director ___ X ___ 10% Owner
_____ Officer (give title below) _____ Other (specify below)

(Street)

CAMBRIDGE, MA 02142

(City) (State) (Zip)

5. If Amendment, Date Original Filed (MM/DD/YYYY)

6. Individual or Joint/Group Filing (Check Applicable Line)

___ Form filed by One Reporting Person
_ X _ Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Beneficially Owned
1.Title of Security (Instr. 4)	2. Amount of Securities Beneficially Owned (Instr. 4)	3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	4. Nature of Indirect Beneficial Ownership (Instr. 5)
Common Stock	4444444 ⁽¹⁾	I	See Footnote ⁽²⁾
Common Stock	4444444 ⁽³⁾	I	See Footnote ⁽⁴⁾

*Table II - Derivative Securities Beneficially Owned ( e.g.* , puts, calls, warrants, options, convertible securities)**
1. Title of Derivate Security (Instr. 4)	2. Date Exercisable and Expiration Date (MM/DD/YYYY)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
1. Title of Derivate Security (Instr. 4)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	4. Conversion or Exercise Price of Derivative Security		6. Nature of Indirect Beneficial Ownership (Instr. 5)

Explanation of Responses:
(1)	On June 18, 2019, Flagship Pioneering Fund VI, L.P. ("Flagship Fund VI") acquired 4,444,444 shares pursuant to an underwritten public offering of common stock by the issuer (the "Offering").
(2)	The shares are held by Flagship Fund VI. Flagship Pioneering Fund VI General Partner LLC ("Fund VI GP") is the general partner of Flagship Fund VI. Flagship Pioneering, Inc. ("Flagship Pioneering") is the manager of Fund VI GP. Noubar B. Afeyan Ph.D. is the CEO and sole shareholder of Flagship Pioneering and may be deemed to have sole voting and investment power with respect to all shares held by Flagship Fund VI. Each of the reporting persons disclaims beneficial ownership of such shares except to the extent of its pecuniary interest therein.
(3)	On June 18, 2019, Nutritional Health LTP Fund, L.P. ("Nutritional LTP") acquired 4,444,444 shares pursuant to the Offering.
(4)	The shares are held by Nutritional LTP. Nutritional Health LTP Fund General Partner LLC ("Nutritional LTP GP") is the general partner of Nutritional LTP. Noubar B. Afeyan Ph.D. is the sole member and manager of Nutritional LTP GP and may be deemed to have sole voting and investment power with respect to all shares held by Nutritional LTP. Each of the reporting persons disclaims beneficial ownership of such shares except to the extent of its pecuniary interest therein.

Reporting Owners
Reporting Owner Name / Address	Relationships
Reporting Owner Name / Address	Director	10% Owner	Officer	Other
Flagship Pioneering Inc. 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA 02142		X
Flagship Pioneering Fund VI General Partner LLC 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA 02142		X
Flagship Pioneering Fund VI, L.P. 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA 02142		X
Nutritional Health LTP Fund General Partner LLC 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA 02142		X
Nutritional Health LTP Fund, L.P. 55 CAMBRIDGE PARKWAY CAMBRIDGE, MA 02142		X

Signatures
Flagship Pioneering, Inc., By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: CEO		6/28/2019
^** Signature of Reporting Person		Date
Flagship Pioneering Fund VI General Partner LLC, By: Flagship Pioneering, Inc., its manager, By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: CEO		6/28/2019
^** Signature of Reporting Person		Date
Flagship Pioneering Fund VI, L.P., By: Flagship Pioneering Fund VI General Partner LLC, its general partner, By: Flagship Pioneering, Inc., its manager, By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: CEO		6/28/2019
^** Signature of Reporting Person		Date
Nutritional Health LTP Fund General Partner LLC, By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: Manager		6/28/2019
^** Signature of Reporting Person		Date
Nutritional Health LTP Fund, L.P., By: Nutritional Health LTP Fund General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Name: Noubar B. Afeyan, Ph.D., Title: Manager		6/28/2019
^** Signature of Reporting Person		Date

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
*	If the form is filed by more than one reporting person, see Instruction 5(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).
Note:	File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
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