The influence of sensory input on the locomotor output of spinal preparations of the dogfish, Scyliorhinus canicula, was studied under conditions where the phasic sensory input was reduced, abolished or modified. A reduction in the input, produced by decreasing the amplitude of the swimming movements by body cooling and by weakening injections of curare, was correlated with a slowing of tail-beat frequency. Suction electrode recordings from segmental motor nerves in swimming fish, that were subsequently paralysed, indicated that the onset of paralysis and the reduction in movement amplitude was accompanied by a rapid decrease in the frequency of the motor rhythm. A rhythmical motor output was still generated when all movements ceased. The co-ordination of this motor output was compared with that observed in the swimming fish over a wide range of output frequencies and found to differ only at very slow output frequencies. Phasic sensory input was reintroduced into paralysed fish by imposing mechanical oscillations onto the body of the fish. Within a range of frequencies these oscillations resulted in the production of synchronized bursts of motor activity from the spinal cord. However, at higher stimulus frequencies there were fewer motor bursts than body oscillations while at lower frequencies there were more motor bursts than oscillations. Recordings obtained from segmental sensory nerves during body oscillation showed that body sense organs discharged neural bursts in time with the oscillation. It was found that the phase of the sensory input remained constant with increasing frequencies of movement; this is in contrast to the phase of the motor output which increased with increasing frequency. These results support the view that the spinal cord contains a self-sustaining neural network which can be entrained by phasic sensory input; the extent of this entrainment, however, is limited by intrinsic properties of the network.