The nucleotide sequence of the cDNA coding for Ayu somatolactin (SL), a fish-specific pituitary hormone belonging to the growth hormone (GH)/prolactin (PRL) family, has been determined. The full length Ayu SL cDNA (1073bp) encodes a protein of 237aminoacids (aa), with a putative signal peptide of 25 aa and a mature protein of 212 aa. Ayu SL contain seven Cys residues found to be characteristic of SLs, and one potential N-glycosylation site. A high sequence homology between Ayu and nine other fish SLs reveal SL to be highly conserved. Using RT-PCR SL showed the highest level of expression in pituitary with light to moderate expression in other body parts. Comparison of sequences showed higher identity of SL to GH than PRL.