The nation's three largest credit unions still choose to do their data processing on their own in-house. In fact, seven of the top 50 CUs go that route. Putting them aside, Symitar Systems has the largest slice of the big credit union market with 14 clients. They are followed by Open Solutions Inc. with eight, XP Systems and Harland Financial Services each with five, Summit with four, Fidelity National with three, and USERS, EPL, IntegraSys and Aftech each with one of the top 50. The following list, provided by Callahan & Associates, lists the largest CUs along with their data processing vendor.