Neuropeptide F (NPF), originally isolated from the sheep tapeworm, Moniezia expansa, consists of 39 amino acid residues terminating in a phenylalaninamide. An analogous neuropeptide has been isolated and sequenced from extracts of circumoesophageal ganglia of the garden snail, Helix aspersa. This neuropeptide exhibits partial primary structural similarity to members of the vertebrate neuropeptide Y (NPY)/pancreatic polypeptide (PP) superfamily. NPF is thus of widespread occurrence in the nervous systems of invertebrates from different phyla and may represent the phylogenetic precursor of the vertebrate NPY/PP superfamily.