Characterization of Atlantic salmon serum transferrin cDNA representing a near full-length transferrin mRNA revealed a 2,070-bp open reading frame encoding a protein of 690 amino acids. The predicted protein contains an 18 amino acids' long signal sequence and shows 49% amino acid positional identity with Xenopus laevis transferrin and human serum transferrin. On the basis of sequence differences obtained from different salmon transferrin cDNA clones, 2 distinct classes of transferrin mRNA were identified. Both variants were present together in genomic DNA from haploid embryos, demonstrating that Atlantic salmon have 2 transferrin genes per haploid complement. Salmon transferrin is expressed mainly in the liver.