This paper proposes a novel scheduling discipline for service differentiation in multi-service networks, which is referred to as the Probabilistic Priority (PP) discipline. PP is based on the Strict Priority (SP) discipline with the difference that each priority class is assigned a parameter. The parameter determines the probability with which its corresponding queue is served when it is polled by the server. Service differentiation as well as fairness among traffic classes can be achieved in PP by setting the assigned parameters properly. In addition, PP can be easily reduced to the ordinary SP or to the reverse SP. Moreover, PP can provide service segregation among groups of traffic classes while at the same time provide service differentiation among classes within each group.