This paper describes the move from a propictary mainframe application to open systems application running the 360 "Fares system shose business logic omponent is implemented in prolog. It is one of the hargest and most profitable Prolog applications written. Prolog ia is the business-rulw component application that includes network, user interface security data access tiers. There has been no downtime, scheduled or unscheduled, in the 360 "Fares system since June 2004