Insulin-like growth factor I (IGF-I) plays a major role in development and metabolism. Currently, the cDNA-derived primary structure of IGF-I is known for some mammals and for chicken, frog, and salmon. Additionally, the organization of the human, rat, and chicken IGF-I genes has been established. The investigation of IGF-I gene structure in fish would extend the evolutionary picture for this hormone and facilitate our understanding of the features of the IGF-I gene that are common to all vertebrate species. The cloned chum salmon IGF-I gene appears to be much more compact than the mammalian and avian genes, being less than 20 kb in length. As in other species, however, the mature IGF-I peptide appears to consist of 70 amino acids and is encoded by exons 2 and 3. Intriguingly, exon 1-encoded 5'-untranslated region sequences are highly conserved, while the coding sequences at the 3' end of the same exon are less conserved. The amino terminus of the signal peptide is four amino acids shorter than in the mammalian and avian peptides. The end of the B domain, the C, A, and D domains, and the first part of the E peptide are encoded by exon 3, but the exon 3-encoded E peptide sequence is 27 amino acids longer than in other species. These extra 27 amino acids, encoded by both coho and chum salmon cDNAs, may be deleted by alternative splicing, as suggested from the sequence of a coho salmon IGF-I cDNA.(ABSTRACT TRUNCATED AT 250 WORDS)