Using skeletochronology, the age composition and growth rates of Pelophylax ridibundus population from the Eastern European part of the native range (Zvenigorod Biological station (ZBS) of Lomonosov Moscow State University, Moscow region) were compared with introduced populations of Kamchatka, differing in the duration of the activity season (T). Both females and males of the ZBS population (T = 5.5 months) reach maturity after the 3rd wintering.