Sec Form 4 Filing - CHUNG PETER Y @ Acacia Communications, Inc. - 2019-02-26

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 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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1. Name and Address of Reporting Person CHUNG PETER Y 2. Issuer Name and Ticker or Trading Symbol Acacia Communications, Inc. [ ACIA] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O SUMMIT PARTNERS, 222 BERKELEY STREET, 18TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 02/26/2019 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ 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 (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 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 02/26/2019 S 1,300,127 ( 1 ) D $ 52.25 10,590 ( 2 ) I See Footnotes ( 3 ) ( 4 )

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 (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 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 (MM/DD/YY) 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

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other CHUNG PETER Y C/O SUMMIT PARTNERS 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Signatures Robin W. Devereux, POA for Peter Y. Chung 02/28/2019 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents shares sold by the following entities: 987,039 shares sold by Summit Partners Venture Capital Fund III-A, L.P., 299,157 shares sold by Summit Partners Venture Capital Fund III-B, L.P., 12,860 shares sold by Summit Investors I, LLC and 1,071 shares sold by Summit Investors I (UK), L.P. ( 2 )Consists of 4,446 shares and 6,144 restricted stock units held by Peter Y. Chung for the benefit of Summit Partners, L.P. ( 3 )Summit Partners, L.P. is the managing member of Summit Partners VC III, LLC, which is the general partner of Summit Partners VC III, L.P., which is the general partner of each of Summit Partners Venture Capital Fund III-A, L.P. and Summit Partners Venture Capital Fund III-B, L.P. Summit Master Company, LLC is the managing member of Summit Investors Management, LLC, which is the manager of Summit Investors I, LLC, and the general partner of Summit Investors I (UK), L.P. Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated investment decisions, including voting and dispositive power, to Summit Partners, L.P. and its investment committee responsible for voting and investment decisions with respect to the Issuer. ( 4 )Summit Partners, L.P., through a two-person investment committee responsible for voting and investment decisions with respect to the Issuer, currently comprised of Martin J. Mannion and Peter Y. Chung, has voting and dispositive power over the shares held by each of these entities and therefore may be deemed to beneficially own such shares. Mr. Chung is a member of Summit Master Company, LLC. Each of the entities listed in this report, Mr. Mannion and Mr. Chung expressly disclaims beneficial ownership of such shares except to the extent of their pecuniary interest therein.

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