Commissural interneurons in the lamprey coordinate activity of the hemisegmental oscillators to ensure proper left-right alternation during swimming. The activity of interneuronal axons at the ventral commissure was studied together with potential target motoneurons during fictive locomotion in the isolated lamprey spinal cord. To estimate the unperturbed activity of the interneurons, axonal recordings were chosen because soma recordings inevitably will affect the level of membrane depolarization and thereby spike initiation. Out of 227 commissural axons recorded during locomotor activity, 14 produced inhibitory and 3 excitatory PSPs in target motoneurons. The axons typically fired multiple spikes per locomotor cycle, with around 10 Hz sustained frequency. The average shortest spike interval in a burst corresponded to an instantaneous frequency of around 50 Hz for both the excitatory and inhibitory axons. The maximum number of spikes per locomotor cycle was inversely related to the locomotor frequency, in accordance with previous observations in the spinal hemicord preparation. In axons which fired multiple spikes per cycle, the mean inter-spike intervals were in the range in which the amplitude of the slow afterhyperpolarization (sAHP) is large, providing further support for the role of the sAHP in spike timing. 195 axons (86%) fired rhythmically during fictive locomotion, with preferred phase of firing distributed over either the segmental locomotor burst phase (40% of axons), or the transitional phase (between bursts; 60%). Thus, in lamprey commissural interneurons, we found a broad distribution of firing rates and phases during fictive locomotion.