The size of sexual maturity, seasonal changes in the body condition and feeding habits of the pleuronectid flounder Kareius bicoloratus were studied from June 1989 to June 1991 in samples taken from Tokyo Bay. The spawning period was estimated to occur from December to February-March from the seasonal changes in the gonadosomatic index of both sexes. The minimum size of sexual maturity for males and females collected in this study were 130 mm and 183 mm standard length, respectively. The percentage of mature fish at age 0 differed between sexes: 69.1% for males and 4.5% for females. For age-1 and older fish, the percentage was about 100% in both sexes. From the seasonal changes in the condition factor and the hepatosomatic index, it appears that the condition of adult fish was at the lowest level in winter. Thereafter, the condition of fish reached the highest level in spring with one or two months delay from when the feeding intensity was highest, and declined throughout the summer to winter. The feeding intensity lessened during the spawning period, especially for the fish which had a high gonadosomatic index. Dominant food items varied seasonally and were mainly composed of polychaetes, bivalves, crustaceans and ophiuroids.