We isolated a cDNA encoding ribosomal protein S2 in sockeye salmon, Oncorhynchus nerka, using a reverse transcriptase-polymerase chain reaction (RT-PCR) method. The cDNA encoding ribosomal protein S2 is composed of 933 nucleotides, and has a 5'-noncoding sequence of 9 bases, a 885 base open reading frame coding for a 294 amino acid polypeptide, and a 39 base 3'noncoding sequence. The amino acid sequence of sockeye salmon S2 protein deduced from the nucleotide sequence is highly homologous to those from the rat (86.1%) and Drosophila melanogaster (73.6%). The N-terminal region of S2 protein is rich in arginine-glycine sites, including eight tandem repeats, and has two consecutive copies of the RGGF motif. The sequences are considered to be requisites for nucleolar localization and binding to RNA for nucleolar proteins. Southern blot analysis indicates that there may be only a single copy of the S2 gene, which is a multiple copy gene in the rat and the fruit fly. Northern blot analysis shows that the S2 gene is expressed in the brain, pituitary, heart, liver kidney, muscle, testis and ovary of sockeye salmon.