1. Large neurons in the pedal ganglia of Aplysia californica were examined for their potential to modulate motoneuron-evoked contractions in foot and body-wall muscle. These neurons lie near the medial border of each pedal ganglion and have peripheral axons but no detectable motor effect. 2. Neurons in this region fire in rhythmic bursts during fictive escape crawling, and their action potentials resemble those recorded from parapodial opener-phase (POP) neurons in the swimming species, A. brasiliana. 3. Firing these neurons in conjunction with pedal motoneurons potentiates the force of contractions and increases their relaxation rate. Similar effects are produced by the serotonin (5-HT) analogue, bufotenine. 4. The modulatory neurons can be stained in vivo by 5,7-dihydroxytryptamine (5,7-DHT), suggesting they are serotonergic. 5-HT immunoreactivity is present in axons associated with foot and body-wall muscle. 5. Bath-applied 5-HT causes rhythmic bursting in the modulatory neurons. It appears likely that they are homologous to the POP cells of A. brasiliana.