The incidence of ovigerous females in populations of two grapsid crabs, Hemigrapsus penicillatus (de Haan, 1835) and Sesarma (Parasesarma) pictum (de Haan, 1835) were followed from August 1975 to November 1976. H. penicillatus, which inhabits the lower intertidal region near the mouth of Tatara-Umi Estuary, breeds from March to November. S. pictum, which inhibits crevices and abandoned holes of other species and is abundant at and above the upper intertidal region of the estuary, breeds from May to September. H. penicillatus matures when the female reaches 6 to 7 mm carapace width, whereas S. pictum becomes mature when the carapace width is 12 to 13 mm, although the maximum size attained by females of both species is almost the same. H. penicillatus produces 5 to 6 broods, S. pictum 2 to 3 broods, during a breeding season. The size of a newly laid egg of H. penicillatus is smaller than that of S. pictum. The number of eggs produced by a female H. penicillatus is greater than that of S. pictum during a breeding season. In both species the peak of the breeding season is during summer, at which time the ovarian activity is also apparently accelerated. The major environmental factor which controls the breeding in these crabs appears to be temperature. H. penicillatus is submerged at every high tide, and is relatively inactive from late November to February when the ambient water and air temperatures are rather low. S. pictum is dormant in crevices or understones of the splash zone from November to March. The length of the breeding season of these crabs appears to be inversely proportional to the period of their winter dormancy.