Sensor-actuator networks (SANs) are a new approach for the physically-based animation of objects. The user supplies the configuratíon of a mechanical system that hás 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 bythis method are presented. CR Categorias: G.3 [Probabüity and Statistics]: Probabilistic