Exhibit 5.1

                              OPINION OF JONES DAY



                                                  March 11, 2004


Transaction Systems Architects, Inc.
224 South 108th Avenue
Omaha, Nebraska  68154


              Re:   Registration Statement on Form S-8


Ladies and Gentlemen:

         We are acting as counsel to Transaction Systems Architects, Inc., a
Delaware corporation (the "Company"), in connection with the registration of
750,000 shares (the "Shares") of the Company's Class A Common Stock, par value
$0.005 per share, pursuant to a Registration Statement on Form S-8 to be filed
by the Company on or about March 12, 2004 (the "Registration Statement").

         We have examined such documents, records, and matters of law as we have
deemed necessary for purposes of this opinion. Based on such examination and on
the assumptions set forth below, we are of the opinion that the Shares that may
be issued and sold pursuant to the Company's Amended and Restated 1999 Employee
Stock Purchase Plan (the "Plan") and authorized forms of stock agreements
thereunder will be, when issued and sold in accordance with the Plan and such
agreements, duly authorized, validly issued, fully paid and nonassessable.

         In rendering the foregoing opinion, we have (i) relied as to certain
factual matters upon certificates of officers of the Company and public
officials, and we have not independently checked or verified the accuracy of the
statements contained therein, (ii) assumed that the signatures on all documents
examined by us are genuine and that where any such signature purports to have
been made in a corporate, governmental, fiduciary, or other capacity, the person
who affixed such signature to such documents had authority to do so, and (iii)
assumed the authenticity of all documents submitted to us as originals and the
conformity to original or certified copies of all copies submitted to us as
certified or reproduction copies. In addition, our examination of matters of law
has been limited to the General Corporation Law of the State of Delaware and the
federal laws of the United States of America, in each case as in effect on the
date hereof.

         We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement.

                                                  Very truly yours,


                                                  /s/ Jones Day