The body size and age structure of a breeding population of the Japanese common toad, Bufo japonicus formosus, was studied at Yamakitamachi in Kanagawa Prefecture for three breeding seasons from 1992 to 1994. Yearly variation in snout-vent length (SVL) was not significant, but the difference in SVL between the sexes was highly significant; the mean SVL of males and females was 125.8 and 134.2 mm, respectively. The breeding adults were aged successfully by skeletochronology using phalanges. The breeding adults were 1-8 yr old. The mean and median ages were 4.5 and 4 yr in males, and 5.2 and 5 yr in females, respectively. The males were estimated to begin to breed when one to four yr old, with a mean of 2.4 yr, while the females were estimated to breed when two to four yr old, with a mean of 2.7 yr. Age at first reproduction was significantly younger for males than females. We discuss the sexual size dimorphism of B. j. formosus in relation to the difference in age structure of the breeding population between the sexes clarified by skeletochronology.