The design, development, and performance of CM-RCS, a neural network simulator for the Connection Machine (CM), are described. The design of the simulator is based on the Rochester Connectionist Simulator (RCS) for connectionist networks. The CM-RCS can be used as a stand-alone system or as a high-performance parallel back end to RCS. In the latter case, once the network has been built by RCS, the back-end system constructs and executes an equivalent network on the CM processor array. The CM-RCS facilitates the exploitation of the massive parallelism inherent in connectionist networks. It also enables substantial reduction in the training times of connectionist networks.