The locomotor muscle fibres of the chaetognath Sagitta are probably multiply innervated and, although linked by numerous gap junctions, do not appear to be coupled electrically. Acetylcholine evokes contraction of the locomotor muscle; iontophoretic application of acetylcholine evokes membrane depolarizations and a series of spikes resembling those seen during spontaneous activity. Both effects are reversibly abolished by (+)-tubocurarine. Acetylcholinesterase is found associated with the sarcolemmata, and it is suggested that acetylcholine is a possible candidate for the neuromuscular transmitter. Immunocytochemical studies with antisera raised against the sequence Arg-Phe-amide (RFamide) show that many neurons in the brain and ventral ganglia contain an RFamide-like material. Some of these neurons are associated with the innervation of sensory organs; others may belong to coordinating systems. At present, these observations do not indicate the affinity of the Chaetognatha to any other invertebrate phylum.