The neuroactive peptide Phe-Met-Arg-Phe-NH2 (FMRF-amide) has a variety of effects on both mammalian and invertebrate tissues; moreover, FMRFamide-like immunoreactivity is found throughout the animal kingdom. Here we describe the isolation and characterization of a cDNA clone from an Aplysia abdominal ganglion cDNA library that encodes a precursor protein that may give rise to as many as 19 individual FMRFamide peptides. Nearly all of the FMRF sequences are flanked on the amino terminus by Lys-Arg residues and on the carboxy terminus by Gly-Lys residues, suggesting that the single lysine residues function to signal cleavage by processing enzymes. The gene is present in a single copy per haploid genome and gives rise to multiple transcripts, at least some of which appear to arise through alternate RNA splicing. Immunohistochemical analysis suggests that the peptide is present in many neurons throughout the Aplysia nervous system and that these neurons send processes to a variety of different tissues.