Using a radioimmunoassay for the peptide sequence Arg-Phe-NH2, a peptide has been purified from acetic acid extracts of the sea anemone Anthopleura elegantissima. This peptide has the structure less than Glu-Ser-Leu-Arg-Trp-NH2. Using antisera to its carboxyterminal sequence Arg-Trp-NH2, the peptide was found to be exclusively localized in neurons of sea anemones, among them neurons associated with the sphincter muscle. This suggest that the peptide is a transmitter at neuromuscular junctions.