This study focuses on the life history of the Mediterranean green crab Carcinus aestuarii along the northern shores of Tokyo Bay, Japan. Hand collection, baited traps and artificial cobble habitats were used over a period of two years at two sites, one along a canal and another in a man-made lagoon. Planktonic larvae were also surveyed. Collected crabs were analyzed for size, sex, molting condition, and ovigerous females were recorded. The results showed that the peak of megalopal settlement occurs in March. These settlers (crabs) then grow steadily throughout the summer months, and most females are able to breed during their first year. In autumn, the crabs migrate offshore, where they stay until the following spring. The breeding season lasts from November to May. Survival of larvae released late in the season, however, is lower, most likely due to increased fish predation and also to bottom hypoxia. Size analysis indicates that the crabs live at least three years, and most are able to breed three times during that period. Females, however, appear to be less likely than males to overwinter twice or more. The seasonal patterns of breeding and offshore migration observed here corresponds well with that documented for the Mediterranean green crabs in their native habitat. Since severe bottom hypoxia develops during the summer months in the offshore areas of upper Tokyo Bay, the seasonal migration pattern allows the crabs to avoid this hypoxia while utilizing wide areas of the bay during the autumn and winter months, when oxygen levels have recovered. In addition, the winter breeding habit allows most of the larvae to develop, migrate inshore and settle before predation and early bottom hypoxia become obstacles.