Field observations on the population trends and life history of recently introduced spider crab Pyromaia tuberculata were made in organically polluted inner Tokyo Bay. The crab population was abundant through late fall to early summer at the deeper subtidal bottom where heavy oxygen deficiency and temporal extinction of benthic animals occurred every late summer. Egg-carrying females were found throughout the year. By aquarium experiments, negative logarithmic relationship was ascertained between incubation period of eggs and the water temperature. The incubation period varied from 7 days (26°C, summer) to 84 days (8.5°C, winter). The recruitment of young crabs was recorded in early spring, midsummer and late fall, but successful recruitment occurred only in late fall when the bottom environmental conditions improved and the adult density was low. Young crabs recruited in late fall were estimated to reach maturity and start breeding within three months. Adaptive significance of such life history traits of the crab to subsistence under organically pollluted unstable environment is discussed.