The nucleotide sequence of chum salmon preproinsulin gene isolated from the phage library of genomic sequences is presented. The transcription initiation site of the gene was experimentally determined. The resolved upstream region contains a TATA-promoter sequence and CAAT-like-box sequence. The salmon insulin gene is split and contains two introns. The first intron, 393 b.p. long, is situated in the 5'-untranslated region. The second intron, 287 b.p. long, interrupts triplet coding for the seventh amino acid residue of the C-peptide. Thus, the overall structure of the insulin gene appears to be closely related to other sequenced insulin genes, including those of birds and mammals. Both introns in the salmon insulin gene are flanked by GC...AG pairs in contrast to GT...AG, the rule common for the majority of genes. Analysis of the cloned salmon insulin gene and cDNAs reveals a high frequency of polymorphic differences, up to 6% in the translated region. The nucleotide sequence of the 3'-untranslated region of the gene is only 70% homologous to the corresponding region in the previously cloned salmon preproinsulin cDNAs. These differences may implicate for the presence of more than one insulin gene per haploid genome in salmon.