Using a radioimmunoassay against the C-terminal sequence Arg-Pro-NH₂ (RPamide) we have isolated the neuropeptide <Glu-Asn-Phe-His-Leu-Arg-Pro-NH₂ (Antho-RPamide II) from extracts of the sea anemone Anthopleura elegantissima. Addition of low concentrations of Antho-RPamide II to a tentacle preparation of sea anemones inhibited the spontaneous, rhythmic contractions, suggesting that the peptide is a neurotransmitter or neuromodulator.