EXHIBIT 5.1



                                  April 3, 2000


APA Optics, Inc.
2950 N.E. 84th Lane
Blaine, MN 55449

         Re:      Registration Statement on Form S-3

Ladies and Gentlemen:

         We have acted as counsel for APA Optics, Inc. (the "Company") in
connection with a Registration Statement on Form S-3 (the "Registration
Statement") that relates to the issuance of up to 250,000 shares of common stock
of the Company upon conversion of the Company's 2% Series A Convertible
Preferred Stock and up to 57,500 shares upon exercise of related warrants for
purchase of common stock (collectively, the "Shares").

         In connection with the registration of the Shares, we have examined
such documents, records and matters of law as we have deemed necessary to the
rendering of the following opinion. Based upon that review, it is our opinion
that upon receipt of payment in full therefor, the Shares being registered, when
issued by the Company, will be validly issued, fully paid and non-assessable.

         The foregoing opinion is subject to the qualification that the Shares
or any of them, when issued, will not when taken together with the then issued
and outstanding capital stock of the Company, exceed the authorized capital
stock of the Company.

         We consent to the use of this opinion as an exhibit to the Registration
Statement and further consent to the use of our name wherever it appears in the
Registration Statement, including the prospectus constituting a part thereof,
and any amendments thereto.

                                       Very truly yours,

                                       MOSS & BARNETT,
                                       A Professional Association


                                       /s/ Janna R. Severance
                                       Janna R. Severance