The abundance, sex ratio, growth, sexual maturity, morphological sex dimorphism, and reproduction of the portunid crab Charybdis bimaculata was studied in Tokyo Bay, Japan, between November 2002 and October 2003. Relative abundance [inferred from catch-per-unit-effort (CPUE)] was higher and lower during summer and winter months, respectively. The proportion of males varied between 0.2-0.5 throughout the year and the overall sex ratio was greatly biased toward females. The population had a unimodal size frequency distribution and the life-span was estimated to be about one year. Growth was not depressed in winter and was adequately described by the von Bertalanffy growth equation. The puberty molt in males brought about changes in the relative size of the chelipeds and gonopods and was estimated to occur within a carapace length (CL) range of 12.50-16.00 mm. Puberty in females was evidenced by a sharp increase in the pleon width and was estimated to occur at CLs between 11.34 and 16.74 mm. Morphological sexual dimorphism was observed in all features which showed secondary sexual development (cheliped dimensions, carapace and pleon width, and body weight). Ovarian development was uniformly arrested only during winter but began developing in early spring, and mating (insemination) increased in frequency between spring and summer. Ovigerous females were found in all seasons except for winter, but were more common during the warmer months. Batch fecundity ranged from 8,300 to 38,400 eggs per female and was positively correlated with body size. C. bimaculata displays the typical reproductive features and spawning pattern of temperate portunid crabs in spite of its short life-span.