Polymorphism of serum transferrins was found in four of five species of scombroid fishes studied. Evidence for genetic control of these polymorphisms was obtained from tests of fit with the Hardy-Weinberg distribution. Three transferrin phenotypes were present in the yellowfin tuna (Thunnus albacares) and the bonito (Sarda chiliensis). The data for these species fit a two-allele system. Six phenotypes were found in the skipjack (Katsuwonus pelamis) and albacore (T. alalunga), and the data for these species fit a three-allele system. The transferrin of the bluefin tuna (T. thynnus) was not polymorphic. The usefulness of differences in transferrin gene frequencies for population studies, especially of the yellowfin and skipjack, is discussed.