For the investigation of the evolution of the proopiomelanocortin (POMC) gene in early ray-finned fishes, nucleotide sequence of POMC cDNA from a chondrostean fish, the sturgeon has been determined. POMC cDNA was amplified by PCR from double-strand cDNA prepared from sturgeon pituitary and ligated with lambdaZAP II. The POMC cDNA consists of 1079 bp without a poly-A. An open reading frame of the POMC cDNA encodes 263 amino acid residues. Sturgeon POMC contains ACTH, alpha-melanotropin (MSH), beta-MSH and beta-END at positions (115-153), (115-127), (186-202) and (205-238), respectively. Location of POMC(51-72) is homologous to gamma-MSH, whereas the third residue of MSH-core sequence, His-Phe-Arg-Trp, is changed to His. Moreover, there are no basic amino acids to serve as a processing signal on the N-terminal side of POMC(51-72). These structural characteristics suggest that an ancestor of the ray-finned fishes had gamma-MSH, whereas significant mutations occurred during the evolution of chondrostean fish.