<PAGE>

                                                                     Exhibit 5.1

                                August 31, 2001



Applied Extrusion Technologies, Inc.
3 Centennial Drive
Peabody, Massachusetts  01960

     Re:  $275,000,000 aggregate principal amount of 10-3/4% Series B Senior
          Notes due 2011 of Applied Extrusion Technologies, Inc. issued in
          exchange for $275,000,000 aggregate principal amount of 10-3/4% Senior
          Notes due 2011 of Applied Extrusion Technologies, Inc.

Ladies and Gentlemen:

     We have acted as counsel to Applied Extrusion Technologies, Inc., a
Delaware corporation (the "Company"), and Applied Extrusion Technologies
(Canada), Inc., a Delaware corporation (the "Guarantor") in connection with (i)
the issuance by the Company in an exchange offer (the "Exchange Offer") of
$275,000,000 aggregate principal amount of its 10-3/4% Series B Senior Notes due
2011 (the "Exchange Notes"), which are to be registered under the Securities Act
of 1933, as amended (the "Securities Act"), in exchange for $275,000,000
aggregate principal amount of the Company's outstanding 10-3/4% Senior Notes due
2011 (the "Initial Notes"), which have not been, and will not be, so registered,
and (ii) the preparation of the registration statement on Form S-4, and
Amendment No. 1 and Amendment No. 2 thereto, filed by the Company with the
Securities and Exchange Commission on the date hereof (the "Registration
Statement") for the purpose of registering the Exchange Notes under the
Securities Act of 1933 (the "Act"). The Initial Notes have been, and the
Exchange Notes will be, issued pursuant to an Indenture dated as of June 19,
2001 (the "Indenture") between the Company, the Guarantor
and Wells Fargo Bank Minnesota, National Association, as trustee (the
"Trustee"). Capitalized terms defined in the Indenture and not otherwise defined
herein are used herein with the meanings so defined.

     This opinion is furnished in accordance with the requirements of Item
601(b)(5) of Regulation S-K under the Act.
<PAGE>

Applied Extrusion Technologies, Inc.   -2-                    August 31, 2001

     We have examined such documents and made such other investigation as we
have deemed appropriate to render the opinions set forth below.  As to matters
of fact material to our opinions, we have relied, without independent
verification, on representations made in the Indenture, certificates and other
documents and other inquiries of officers of the Company and of public
officials.

     The opinions expressed below are limited to matters governed by the laws of
The Commonwealth of Massachusetts, the General Corporation Law of the State of
Delaware, the laws of the State of New York and the federal laws of the United
States of America.

     Based upon the foregoing, we are of the opinion that:

     1.   When the Exchange Notes have been duly executed and authenticated in
accordance with the terms of the Indenture and have been delivered against
receipt of the Initial Notes surrendered in exchange therefor upon completion of
the Exchange Offer, the Exchange Notes will be entitled to the benefits of the
Indenture and will constitute legal, valid and binding obligations of the
Company, enforceable against the Company in accordance with their terms, except
as may be limited by (a) bankruptcy, insolvency, reorganization, moratorium,
fraudulent conveyance and other similar laws relating to or affecting the rights
and remedies of creditors or secured parties generally and (b) general
principles of equity (regardless of whether considered in a proceeding in equity
or at law).

     2.   The Guarantee has been duly authorized, executed and delivered by the
Guarantor in accordance with the terms of the Indenture. The Guarantee
constitutes a legal, valid and binding obligation of the Guarantor, enforceable
against the Guarantor in accordance with its terms, except as may be limited by
(a) bankruptcy, insolvency, reorganization, moratorium, fraudulent conveyance
and other similar laws relating to or affecting the rights and remedies of
creditors or secured parties generally and (b) general principles of equity
(regardless of whether considered in a proceeding in equity or at law).

     We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as an exhibit to the Registration Statement.  We also
consent to the reference to our firm under the caption "Legal Matters" in the
Registration Statement.  In giving this consent, we do not thereby admit that we
are included in the category of persons whose consent is required under Section
7 of the Act or the rules and regulations of the Securities and Exchange
Commission.

                                   Very truly yours,

                                   /s/ Ropes & Gray

                                   Ropes & Gray

