Size composition and growth pattern of the Japanese mitten crab Eriocheir japonica (de Haan, 1835), Varunidae, in their natural habitat was investigated. Crabs were collected using hand-held nets in the Saigo River and the adjacent seacoast in Fukuoka Prefecture, Japan. The dynamics of carapace width (CW) frequency distribution were analyzed and cohorts were identified from polymodal size distribution using Bhattacharya's method. In the lower tidal river and seacoast (Area I, reproductive area of the crabs), adult crabs comprising at least four cohorts were collected. From monthly data for 29 months (January 1997-May 1999) collected from the upper tidal to the lower freshwater area (Area II, growth area of the crabs), two cohorts that had settled in different seasons (fall and early summer) were confirmed for each year. Both cohorts reached minimum adult size (ca. 35 mm < CW and 45 mm < CW, average) in fall 2 years after settlement. This suggests that adult crabs in the tidal area (Area I) mostly comprise two cohorts of 2-year-olds (ca. 24 and 29 months) and two cohorts of 3-year-olds (ca. 36 and 41 months) after settlement.