Body size and age structure were studied for two breeding populations of the salamander, Hynobius tokyoensis, in the southern Kanto plain of Honshu, Japan. The mean snout-vent lengths (SVLs; measured from the tip of snout to the posterior end of cloaca) of males and females were 64.1 and 66.5 mm at Habu in Tokyo, and 63.1 and 65.3 mm at Isumi in Chiba prefecture, respectively. The differences in SVL were not significant between sexes or populations. The breeding adults, successfully aged by skeletochronology using phalanges, were 4–21 years old. The median age was significantly older in the Habu population (eight years) than in the Isumi population (five years), and the age at first reproduction estimated from the age at which the growth of individuals was greatly retarded was two to six years. The modal age at first reproduction was four years for both sexes in both populations except for males at Isumi (three years). In the latter, age at first reproduction was significantly younger than in the others. We discuss the ecological significance of interpopulational variation in age structure clarified by skeletochronology.