Body size and age of two populations of the endangered species Pelophylax porosus porosus from the Kanto Plain, Japan (paddy fields in valley bottoms [Zushi] and open plains [Hayamajima]) were investigated. Age was estimated by skeletochronology of the phalanges. Frogs grew rapidly between 0 and 1 year of age in both populations. The asymptotic snout-vent length (SVL) of both sexes peaked at 2 years. Age at sexual maturity was estimated to be 0–1 years for males, but 1–2 years for females. SVL in adults ranged from 45.2 to 67.2 mm in males, and 60.2 to 88.8 mm in females. In both populations, females were significantly larger than males. However, the SVL was not significantly different between the populations or between age classes in both sexes. Lines of arrested growth (LAGs) were observed in the periosteal tissue. Longevity was estimated to be 3 and 4 years in males and females, respectively. Pelophylax p. porosus is suggested to be the most short-lived and the first to reach sexual maturity compared with the other Pelophylax species in Japan, although the age structure differed between the populations. In both sexes, when comparing the age of 1 year old or older frogs, the Zushi population was significantly older than the Hayamajima population, but the longevity of the Hayamajima population was shorter than that of the Zushi population. As the two study sites differed in agricultural management, it is possible that the resulting environmental conditions influenced the longevity of P. p. porosus.