Aplysia abdominal ganglion neuron L5 is immunoreactive with an antiserum generated against the tetrapeptide Phe-Met-Arg-Phe-amide (FMRFamide); however, the specificity of this immune reagent is limited to the sequence Arg-Phe-amide. We isolated cDNA clones homologous to mRNAs specifically expressed in L5 and demonstrated that these clones do not hybridize to a previously characterized gene encoding FMRFamide. The nucleotide sequence of one of these clones, L5-67, does not encode any FMRFamide peptides but does reveal a Gly-Lys-Arg cleavage site following the amino acids Arg-Phe. This data predicts that neuron L5 expresses a peptide ending in Arg-Phe-amide, consistent with the FMRFamide immunoreactivity.