Intraspecific differentiation of Rana japonica was examined by starch-gel electrophoretic analyses of 15 enzymes and three blood proteins in 505 frogs from 25 populations in Japan. These enzymes and blood proteins were controlled by genes at 25 loci, where 4.6 phenotypes were produced by 3.0 alleles on the average. The fixation indexes (Fst) coined by WRIGHT (1978) were calculated. Of the 25 loci analyzed, the AK and MDH-A loci were zero in Fst, while the Pep-A and Hb-II loci were very high in Fst, being 0.722 and 0.781,respectively. The other 21 loci were 0.020∿0.490 in Fst. The mean proportions of polymorphic loci in each of the 25 populations ranged from 16.00 60.0% with an average of 41.8%, while the mean proportions of heterozygous loci per individual ranged from 4.00 18.1% with an average of 11.3%.0 The mean numbers of alleles per locus ranged from 1.08 to 1.76 with an average of 1.48. Genetic distances among the 25 populations were estimated on the basis of gene frequencies at the 25 loci according to NEI (1975) and a dendrogram was drawn by using the UPGMA clustering method. It was found that Rana japonica first evolved into eastern and western groups. The eastern group was then divided into the Akita population and a subgroup containing eight populations, while the western group was differentiated into the Joetsu population which was followed by the Takatomi and Oki populations and finally branched out into several subgroups containing 13 populations. The genetic distances among the eight populations of the eastern group except the Akita population ranged from 0.005 to 0.100 with a mean of 0.036,while those among 15 populations of the western group except the Joetsu population ranged from 0.014 to 0.159 with a mean of 0.062. The genetic distances between nine populations of the eastern group and 16 populations of the western group ranged from 0.099 to 0.239 with a mean of 0.157.