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                                                                     EXHIBIT 5.1

                               Quarles & Brady LLP
                            411 East Wisconsin Avenue
                         Milwaukee, Wisconsin 53202-4497


                                  June 13, 2000


Plexus Corp.
55 Jewelers Park Drive
P.O. Box 156
Neenah WI 54956

Ladies and Gentlemen:

         We are providing this opinion in connection with the amendment to the
Registration Statement of Plexus Corp. ("Plexus") on Form S-3, No. 33-88095 (the
"Registration Statement"), filed under the Securities Act of 1933, as amended
(the "Act"), with respect to the proposed sale of up to 187,500 shares of Plexus
common stock, $.01 par value (the "Shares"), which are issued in tandem with
preferred stock purchase rights.

         We have examined: (i) the Registration Statement and proposed
amendment; (ii) Plexus' Restated Articles of Incorporation and Restated Bylaws,
as amended to date; (iii) the Merger Agreement pursuant to which the shares were
issued; (iv) corporate proceedings of Plexus and subsidiaries relating to the
Merger Agreement and the transactions contemplated thereby; and (v) such other
documents, and such matters of law, as we have deemed necessary in order to
render this opinion. Based on the foregoing, it is our opinion that:

         1.       Plexus is a corporation duly incorporated and validly existing
                  under the laws of the State of Wisconsin.

         2.       When (a) the Registration Statement, and any amendments
                  thereto (including post-effective amendments), shall have
                  become effective under the Act, and (b) up to 187,500 Shares
                  have been issued in accordance with the provisions of the
                  Rights Agreement, such Shares will have been validly issued
                  and will be fully paid and nonassessable, subject to the
                  personal liability imposed on shareholders by Section
                  180.0622(2)(b) of the Wisconsin Business Corporation Law, as
                  judicially interpreted, for debts owing to employees for
                  services performed, but not exceeding six months service in
                  any one case. Although Section 180.0622(2)(b) provides that
                  such personal liability of shareholders shall be "to an amount
                  equal to the par value of shares owned by them respectively,
                  and to the consideration for which their shares without par
                  value was issued," the Wisconsin Supreme Court, by a split
                  decision without a written opinion, has affirmed a judgment
                  holding shareholders of a corporation liable under the
                  substantially identical predecessor statute in effect prior to
                  January 1, 1991 (Section 180.40(6)) for unpaid employee wages
                  to an amount equal to the consideration for which their par
                  value shares were issued rather than the shares' lower stated
                  par value. Local 257 of Hotel and Restaurant Employees and
                  Bartenders International Union v. Wilson Street East Dinner
                  Playhouse, Inc., 126 Wis. 2d 284, 375 N.W.2d 664 (1985)
                  (affirming the 1983 decision of the Circuit Court for Dane
                  County, Wisconsin, in Case No. 82-CV-0023). The Wilson Street
                  East decision was overruled on other grounds in State v.
                  Richard Knutson, Inc., 191 Wis.2d 395, 528 N.W.2d 430 (1995).




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Plexus Corp.
June 13, 2000
Page 2



         We consent to the filing of this opinion as an exhibit to the amendment
to the Registration Statement and to the reference to our firm under the caption
"Legal Opinions" in the Proxy Statement/Prospectus constituting a part thereof.
In giving our consent, we do not admit that we are "experts" within the meaning
of Section 11 of the Act, or that we are within the category of persons whose
consent is required by Section 7 of the Act or the rules and regulations of the
Securities and Exchange Commission thereunder.


                                Very truly yours,



                                QUARLES & BRADY LLP