Sensor-actuator networks (SANs) are a new approach for the physically-based animation of objects. The user supplies the configuration of a mechanical system that has been augmented with simple sensors and actuators. It is then possible to automatically discover many possible modes of locomotion for the given object. The SANs providing the control for these modes of locomotion are simple in structure and produce robust control. A SAN consists of a small non-linear network of weighted connections between sensors and actuators. A stochastic procedure for finding and then improving suitable SANs is given. Ten different creatures controlled by this method are presented. CR Categorias: G.3 [Probabuity and Statistics]: Probabilistic Algorithms; 1.2.6 [Artificial Intelligence]: Learning, Robotics; 1.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism animation; 1.6.3 [Simulation and Modeling] Applications.