A 37-residue peptide has been purified from the endocrine pancreas of the anglerfish. The first 36 residues were sequenced by gas phase Edman degradation. The sequence at the carboxyl-terminus was determined by sequencing the carboxyl-terminal tryptic dipeptide. The sequence of the peptide, which is consistent with the amino acid composition, was determined to be: Y X P X P X K X P X E X T X P X G X S X N X A X S X P X E X D X W X A X S X Y X Q X A X A X V X R X H X Y X V X N X L X I X T X R X Q X R X Y X G. Fast atom bombardment/mass spectrometry of the peptide identified a molecular ion with an average mass of 4221.3, in good agreement with the theoretical mass based upon the determined amino acid sequence. The peptide has an equal degree of sequence identity to both porcine neuropeptide YY (64%) and gastrointestinal peptide YY (64%), but less sequence identity to porcine pancreatic polypeptide (47%). Unlike the related mammalian peptides, the major form of the anglerfish peptide terminates in tyrosyl-glycine rather than tyrosineamide.