As filed with the Securities and Exchange Commission on November 20, 2012

Registration No.: 333-81184
811-21024

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT
  UNDER THE SECURITIES ACT OF 1933  x

  Pre-Effective Amendment No.  o

  Post-Effective Amendment No. 15  x

and/or

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY
  ACT OF 1940  x

  Amendment No. 17  x

Active Assets Institutional Government Securities Trust

(a Massachusetts business trust)

(Exact Name of Registrant as Specified in Charter)

522 Fifth Avenue
New York, New York 10036

(Address of Principal Executive Office)

Registrant's Telephone Number, Including Area Code: (212) 296-6970

Stefanie V. Chang Yu, Esq.
522 Fifth Avenue
New York, New York 10036

(Name and Address of Agent for Service)

Copy to:

Carl Frischling, Esq.
Kramer Levin Naftalis & Frankel LLP
1177 Avenue of the Americas
New York, New York 10036 Stuart M. Strauss, Esq.
Dechert LLP
1095 Avenue of the Americas
New York, New York 10036

Approximate Date of Proposed Public Offering:

As soon as practicable after this Post-Effective Amendment becomes effective.

It is proposed that this filing will become effective (check appropriate box):

X	Immediately upon filing pursuant to paragraph (b)
	On October 31, 2012 pursuant to paragraph (b)
	60 days after filing pursuant to paragraph (a)(1)
	On (date) pursuant to paragraph (a)(1)
	75 days after filing pursuant to paragraph (a)(2)
	On (date) pursuant to paragraph (a)(2) of Rule 485.

  Amending the Prospectus and Updating Financial Statements

  If appropriate, check the following box:

This post-effective amendment designates a new effective

date for a previously filed post-effective amendment.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of the Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment to the Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York and State of New York on the 20th day of November, 2012.

ACTIVE ASSETS INSTITUTIONAL GOVERNMENT
SECURITIES TRUST
By:	/s/ KEVIN KLINGERT
	Kevin Klingert
	President and Principal
	Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment No. 15 has been signed below by the following persons in the capacities and on the dates indicated.

Signatures			Title	Date
(1)  Principal Executive Officer			President and Principal
			Executive Officer
By:	/s/ KEVIN KLINGERT			November 20, 2012
	Kevin Klingert
(2)  Principal Financial Officer			Chief Financial Officer
By:	/s/ FRANCIS J. SMITH			November 20, 2012
	Francis J. Smith
(3)  Majority of the Trustees
James F. Higgins
By:	/s/ STEFANIE V. CHANG YU
	Stefanie V. Chang Yu
	Attorney-in-Fact			November 20, 2012
	Michael Bozic	Michael E. Nugent (Chairman)
	Manuel H. Johnson	Fergus Reid
	Joseph J. Kearns
By:	/s/ CARL FRISCHLING			November 20, 2012
	Carl Frischling
	Attorney-in-Fact

EXHIBIT INDEX

Index No.	Description of Exhibit
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase