1. Hybridization experiments were performed by using five pond frog species distributed in Japan, Korea and Europe and a Nearctic species in order to clarify their evolutionary relationships. 2. It was confirmed from the results of reciprocal crosses between the Japanese and Korean populations of Rana nigromaculata that the two populations belong to the same species. 3. Two European species, Rana lessonae and Rana ridibunda perezi, are completely isolated from Rana nigromaculata by hybrid inviability, while they are completely isolated from Rana brevipoda by hybrid sterility. 4. The Korean Rana plancyi chosenica is incompletely isolated from both nigromaculata and brevipoda by hybrid sterility. The isolation between chosenica and nigromaculata is somewhat less than it is between nigromaculata and brevipoda, while the isolation between chosenica and brevipoda is greater than it is between the latter. The Korean species is completely isolated by hybrid sterility from the European perezi and incompletely from the European lessonae. 5. The two European species are isolated to a large extent by hybrid sterility in those few crosses in which hybrids were viable. In some cases, they are completely isolated by hybrid sterility. 6. It is presumed that the common ancestor of Palearctic pond frogs was divided into Asian and European branches; the Asian branch diverged into brevipoda, chosenica and nigromaculata, while the European branch diverged into lessonae and perezi. 7. A Nearctic species, Rana pipiens, is completely isolated from the five Palearctic species by hybrid inviability. All the crosses with male or female pipiens result in the death of fertilized eggs at the early gastrula stage.