The young Xenopus tadpole is a very simple vertebrate that can swim. We have examined its behavior and neuroanatomy, and used immobilized tadpoles to study the initiation, production, coordination, and termination of the swimming motor pattern. We will outline the sensory pathways that control swimming behavior and the mainly spinal circuits that produce the underlying motor output. Our recent work has analyzed the glycinergic, glutamatergic, cholinergic, and electrotonic synaptic input to spinal neurons during swimming. This has led us to study the nonlinear summation of excitatory synaptic inputs to small neurons. We then analyzed the different components of excitation during swimming to ask which components control frequency, and to map the longitudinal distribution of the components along the spinal cord. The central axonal projection patterns of spinal interneurons and motoneurons have been defined in order to try to account for the longitudinal distribution of synaptic drive during swimming.