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                                                                EXHIBIT 5.1


                        [Quarles & Brady LLP Letterhead]

                                                                October 23, 2000

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

Gentlemen and Ladies:

         We are providing this opinion in connection with the Registration
Statement of Plexus Corp., a Wisconsin corporation ("Plexus"), on Form S-4 (the
"Registration Statement") being filed under the Securities Act of 1933, as
amended (the "Act"), with respect to the proposed issuance of up to 2,000,000
shares of Plexus Common Stock, $0.01 par value (the "Shares"), in connection
with certain possible future business combination transactions (the
"Transactions"; singularly a "Transaction") pursuant to Rule 415(a)(1)(viii)
under the Act.

         We have examined: (i) Plexus's Articles of Incorporation and Bylaws, as
amended to date; (ii) the Registration Statement, including the form of
Prospectus (subject to completion) contained therein; (iii) corporate
proceedings of Plexus, to date, relating to the proposed issuance of the Shares;
(iv) the Agreement and Plan of Merger dated as of September 29, 2000 among
Plexus, e2E Corporation and Casey Jones Corp. (the "e2E Merger Agreement"); and
(v) such other documents 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 validly existing under the laws of the
              State of Wisconsin.


         2.   When (a) the Registration Statement and any post-effective
              amendment thereto required by General Instruction H to Form S-4
              shall have become effective under the Act, or the information
              necessary in connection with any Transaction is furnished in a
              prospectus supplement in compliance with such General Instruction
              H, (b) any necessary further corporate proceedings shall have been
              taken in connection with the Transactions covered by the
              Registration Statement, as the same may be amended, (c) the
              parties shall have received any necessary regulatory approvals
              required to consummate the Transactions and issue the Shares
              relating thereto, and all other conditions precedent to such
              issuance shall have been satisfied or, where permissible, waived,
              (d) the Transactions shall have been duly consummated in
              accordance with the terms of the definitive agreements relating
              thereto and applicable laws, and (e) up to 2,000,000 Shares have
              been duly issued








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Plexus Corp.
October 23, 2000
Page 3

              in accordance with the provisions of such agreements, such Shares
              will be validly issued, fully paid and non-assessable by Plexus,
              subject to the personal liability which may be 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). That decision was subsequently overturned on
              other grounds.

              We further note that the issuance of shares to the shareholders of
              e2E Corporation pursuant to the e2E Agreement has received the
              corporate approvals contemplated by clause (b) above in this
              paragraph.

         We hereby consent to the use of this opinion as an exhibit to the
Registration Statement and to the reference of our firm under the caption "Legal
Matters" in the Prospectus. As noted therein, attorneys in our firm who are
providing services with respect to the Registration Statement owned an aggregate
of 1,000 shares of Common Stock on October 1, 2000. 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