Lateral ciliary activity was studied on gill preparations of the mussel Mytilus edulis. Electrical stimulation of the branchial nerve accelerated ciliary beating. The cilioexcitatory effect of electrical stimulation of the branchial nerve on the lateral ciliated cell decreased after intramuscular injection of 6-OH-DOPA. Scanning electron microscopic observations of the branchial nerve and visceral ganglion show degeneration of neuronal structures. Therefore, it appears likely that the lateral ciliated cell is dependent on the branchial nerve and visceral ganglion for its neuronal regulation.