Blood specimens of tambaqui, Colossoma macropomum, an Amazonian fish of commercial importance, were examined for transferrin variants by treating blood plasma specimens with rivanol before electrophoretic analyses in starch gel media. A total of 6 co-dominant alleles at the transferrin locus in tambaqui was postulated to account for the 6 varieties of molecular components in 10 observed phenotypes. The population sample showed good genetic balance, characteristic of a unit stock