Coordination of motoneuron activity is a fundamental prerequisite for the generation of functional locomotor patterns. We investigate the neural mechanisms that coordinate activity of motoneuron pools in the vertebrate spinal cord with differing phases of activity in the locomotor cycle in a simple motor system, the lamprey swimming network. In the region of dorsal fins the lamprey spinal cord contains two groups of motoneurons: the myotomal motoneurons that innervate the trunk muscles; and the fin motoneurons controlling muscle fibres of the dorsal fins. We investigated the activity of fin muscles during swimming in vivo and that of fin motoneurons during fictive swimming in vitro. During swimming in vivo with cycle periods of 4-8 Hz, fin muscle activity covered a broad portion of the cycle, with the peak of activity out-of-phase to the ipsilateral myotomal muscles. During fictive swimming evoked by N-methyl-d-aspartate in the isolated spinal cord, fin motoneurons expressed similar out-of-phase activity. The phase relationship of the synaptic drive to fin motoneurons was examined by recording their activity intracellular during fictive swimming. Three different forms of membrane potential oscillation with different time courses in the locomotor cycle could be distinguished. Sagittal lesions of the spinal cord in the segment where fin motoneurons are recorded and up to one segment rostral and caudal from it did not influence the out-of-phase activity pattern of the motoneurons. Our results indicate that coordination of fin motoneuron activity with the locomotor activity of myotomal motoneurons does not depend on intrasegmental contralateral premotor elements.