The physiological and anatomical properties of an unpaired intersegmental interneuron designated cell 205 are described. Cell 205 is unusual among leech neurons in that it combines a variety of functional properties in a single cell. Constant current depolarization of cell 205 initiates and maintains swimming behavior in semi-intact leeches or the swim motor pattern in brainless, isolated nerve cords. During swim episodes elicited by other stimuli, cell 205 is rhythmically active. Current pulses passed into cell 205 during swimming reset the pattern, indicting that it has access to, or may be a member of, the swim central pattern generator (CPG). Cell 205 is the first interneuron in this system to exhibit both swim-initiating and CPG functions. Individual touch, pressure, and nociceptive primary mechanoreceptor neurons polysynaptically excite cell 205, which, in addition, is coupled electrically to the multimodal S interneuron. These inputs may contribute to the initiation and/or modulation of swimming in response to sensory stimuli. Cell 205 shares some common synaptic inputs and outputs with the only other known swim-initiating interneuron, cell 204, but the two cells differ fundamentally in that cell 204 exerts only a tonic effect on the CPG. No synaptic interactions were found between cells 204 and 205, but their excitatory effects on swimming summate. Unlike other swim neurons which are segmentally repeated, cell 205 generally is present only in segment 9, and numerous lines of evidence suggest that it is, in fact, a segmentally differentiated homolog of cell 204.