In order to clarify the phylogenetic relationships of Rana nigromaculata, R. brevipoda, R. plancyi chosenica, R. p. fukienensis, R. lessonae, R. ridibunda and R. esculenta distributed in the Palearctic region, serum transferrins obtained from field-caught animals as well as interspecific hybrids and allotriploids were analyzed by acrylamide-gel electrophoresis. Transferrins of R. nigromaculata were found to have nine phenotypes determined by four alleles, while those of R. p. chosenica had five phenotypes determined by three alleles. R. esculenta had two phenotypes, which were identical to those of the other two European species. Transferrins of R. brevipoda, R. p. fukienensis, R. lessonae and R. ridibunda each showed one phenotype, controlled by one gene. These genes controlled transferrin bands differing in mobility. Bands moved faster in the two R. plancyi subspecies than in the five frog species from Japan and Europe. The electrophoretic patterns of interspecific hybrids and allotriploids completely corresponded to the kind and number of their genes. These findings demonstrated that these frogs belonging to the R. esculenta group were all closely related to each other, and that the two Japanese species, the two Taiwan and Korean subspecies and the three European species composing three subgroups differentiated systematically.