Post-Effective Amendment to Registration Statement by Investment Company (Form 485APOS)

Securities Act of 1933 Registration No. 333-186372

Investment Company Act of 1940 Registration No. 811-22796

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 [X]

[ ] Pre-Effective Amendment No. ______

[X] Post-Effective Amendment No. 40

and

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940 [X]

[X] Amendment No. 40

Fidelity Merrimack Street Trust

(Exact Name of Registrant as Specified in Charter)

245 Summer Street, Boston, Massachusetts 02210

(Address of Principal Executive Offices)(Zip Code)

Registrant's Telephone Number: 617-563-7000

Nicole Macarchuk, Secretary and Chief Legal Officer

245 Summer Street

Boston, Massachusetts 02210

(Name and Address of Agent for Service)

It is proposed that this filing will become effective 83 days after filing pursuant to paragraph (a)(2) of Rule 485 at 5:30 p.m. Eastern Time.