A system and method for replicating a multithreaded application program using a semi-active or passive replication strategy, wherein the application program executes under the control of an operating system having a thread library. The method comprises piggybacking mutex ordering information at the Primary replica onto regular multicast messages specifying the order in which threads in the Primary replica have been granted their claims to mutexes; and receiving the multicast messages at a Backup replica containing the mutex ordering information which determines the order in which threads in the Backup replica are granted mutexes. Thread library interpositioning is preferably utilized to intercept calls to functions in the operating system's thread library, so that the system and method of the invention may be implemented transparently. The invention enforces strong replica consistency without the need to count instructions, add significant messaging overhead, or modify application code.