In this paper; we present the design and implementation of an efficient on-line simulator for wormhole routing networks;, which is an important part of the Concordia Parallel Systems Simulator (CPSS), a simulator for wormhole-routed multicomputers. The ultimate purpose of the CPSS is to provide a parallel programming environment which allows users to study impacts of system and software factors on program performance and to locate performance bottlenecks in parallel programs. Our simulator can accurately simulate a large range of regular topologies that represent the communication structures of most wormhole routed networks. Users are given the flexibility of changing communication and computation parameters as often as needed without recompilation. It is unique in its integrated parallel simulation of the communication process and the execution of user applications. This makes possible real-time evaluation of network performance in realistic application domains.