The occurrence of three green frog taxa [ Rana lessonae (L), R. ridibunda (R), and R. esculenta (E)] in Nizhny Novgorod Province was evidenced by DNA flow cytometry analysis. Our data are the first reliable record of the hybrid R. esculenta for the province (the Middle Volga River basin). All hybrids were diploid. Rana lessonae is spread throughout the province, except its extreme north-east. The distribution of Rana ridibunda is wider and covers the northernmost area. Rana esculenta is quite sparse. In the province, all three species, probably, reach the northern limits of their ranges. Six population systems were revealed; both with a single species (R, L, and, probably, E) and with mixed species populations (LE, REL and RL). The geographic trends in occurrences of hybrids and various kinds of population systems in eastern Europe are discussed.