The N-terminal peptide of salmon proopiocortin has been isolated and the primary structure including two disulfide bonds elucidated. The peptide consisted of 76 amino acid residues, which is 27 residues shorter than the bovine and human peptides. The N-terminal 44 residues of the teleost peptide exhibited significant sequence homology to those of the mammalian peptides. The salmon peptide, however, is lacking in the counterpart of gamma-MSH which is located between residues 51 and 64 in the mammalian peptides.