Davis Polk & Wardwell
450 Lexington Avenue
New York, New York  10017

September 2, 1994

Re:  Defined Asset Funds -
     Municipal Investment Trust Fund,
     Insured Series - 91
     1933 Act Number 33-10654
     1940 Act Number 811-1777

Securities and Exchange Commission
450 Fifth Street, N.W.
Washington, D. C.  20549

Dear Sirs:

Pursuant to Rule 477 under the Securities Act of 1933,
on behalf of Merrill Lynch, Pierce, Fenner & Smith
Incorporated, as agent
for the sponsors of the above referenced trust,
we hereby request you to withdraw the filing pursuant
to Rule 485(b) the
Post-Effective Amendment No. 6 to Form S-6
(the "Registration Statement"), accession number 000095112-94-002100,
which was a duplicate of another filing
on August 10, 1994.

Very truly yours,

Kenneth H. Chase