The body size and age at maturity of female Rana porosa porosa were investigated in two valley bottoms in Tama Hills, Japan. Frogs collected during the breeding season, not showing the male secondary sexual characteristics, were subjected to injection of HCG and were kept with mature males. Individuals that did not ovulate after the dosage were dissected to determine sexes and maturity status on the basis of gonadal morphology. The snout-vent length (SVL) of the smallest mature female was 58.3 mm. The clutch size ranged from 1465 to 2726, and showed a significant positive correlation with SVL. The one-year-old age group of females included both mature and immature individuals, whereas two- and three-year-old females were invariably mature.