Transferrins present in sera of Rana nigromaculata collected from 27 locations in Japan and one location in Korea were analyzed by acrylamide-gel electrophoresis. The results showed that there were 10 different phenotypes controlled by four codominant alleles, a, b, c and d. Of these alleles, allele c was the highest in frequency as a whole and particularly predominant in the Honshu, Chubu, Kinki and Sanin regions. All the four alleles were distributed in the Sanyo and Shikoku regions, and the three other than allele d were found in the Kyushu region and Korea.