This present invention relates to the findings and construction of fish insulin-like growth factor II (IGF-II) cDNAs which can be cloned and expressed in cells. This invention also relates to the production of biologically active fish IGF-II polypeptides by a gene expression system using fish IGF-II cDNAs. The fish IGF-II cDNAs have 1977 bp which transcribe into a peptide (signal peptide), and B,C,A,D,E domain peptides. The fish mature IGF-II is a single polypeptide containing the NH2-B-C-A-D-COOH domains. The mature IGF-II polypeptide is 7 kDa in weight and has 70 amino acids. the fish recombinant IGF-II cDNA can be cloned and expressed in E. coli., yeast, baculovirus, and fish cells.