NeuSim: A modular neural networks simulator for Beowulf clusters. We present a neural networks simulator that we have called NeuSim, designed to work in Beowulf clusters. We have been using this simulator during the last years over several architectures and soon we want to distribute it under GPL license. In this work we offer a detaiied description of the simulator, as well as the performance results obtained with a Beowulf cluster of 24 nodes.