A comparison between two species of Hyla japonica and H. suweonesis in Korea was carried out to clarify their morphological, genetic and ecological differences, to estimate presumed divergent time between them, and to elucidate their probable process and mechanism of speciation. The results are as follows; 1.The breeding season of H. japonica was earlier than that of H. suweonensis about one and half months. 2.H.japonica was distributed all over the country, while H. suweonensis was restricted in Kyonggi plains. 3.On the extemal morphometric characters, differences were hardiy seen between two species. But on the discriminant function analysis, there was apparent differences between them. 4.Genic variation based on 24 loci showed that H. japonica populations were about two fold more variable than that of H. suweonensis populations. 5.Genetic relatedness between two species was rather remote (S =0.520, D=0.634) 6.Their presumed divergent time was estimated to be about three and a quarter mfllon years. It is presumed that H. suweonensis was originated from H. japonica during the period of late pleocene and early pleistocene. 7.Premating isolating mechanisrns (breeding season, mating call, and microhabitat segregation) may be operating to prevent gene ifow between them.