Interneuronal activity was recorded from the spinal cord of paralysed spinal dogfish (Scyliorhinus canicula) showing fictive swimming as indicated by rhythmic activity in the motor nerves. The interneurons from which spike activity was recorded during unstimulated fictive swimming (n = 282) were divided into three groups, according to their firing patterns. Group I units (29%) discharged steadily (mean interspike interval 25-100 ms); their firing patterns were not or only weakly modulated in phase with the spinal cord motor output; when fish with different swimming rhythms were compared, no correlation was found between the average frequency of firing of these units and the mean cycle period of the motor rhythm; when spinal motor output stopped, these neurons remained active. Group II units (19%) discharged throughout the entire cycle of the motor rhythm although a few became phasically active at short cycles; their firing was clearly modulated in line with the motor rhythm and during shorter cycles increased in frequency; when motor output stopped, their spike activity was strongly reduced or absent. Group III units (52%) discharged bursts of action potentials in time with the motor rhythm, each unit firing during its own characteristic phase within the motor output cycle; their firing frequencies increased linearly with locomotor frequency; these units were silent when motor Output stopped.