Reproductive ecology of dinoflagellate, Ceratium furca, was studied in the coastal area of Sagami Bay. Field samplings and laboratory experiments were conducted to investigate seasonal changes of the field population and effects of temperature, salinity and irradiance on the growth rate of C. furca. Abundance of the species increased significantly from April to September and was decreased in November. In particular, the population increased during the spring when the water column was weakly stratified and relatively low nutrient conditions were observed in the surface layer. High growth rates of C. furca were observed at the conditions of 20-28℃, 17-34 PSU and 216-800μEm^s^ with the highest growth rate (μ=0.72d^) being observed at 24℃, 30 PSU and 600μEm^s^. In addition, the growth rates increased gradually with increasing irradiance from 58 to 216μEm^s^ in the all salinity conditions, and afterwards the rates reached plateaus between 216 and 796μEm^s^. The field survey and laboratory experiments indicated that the species is distributed throughout the year and adapted to a wide-range of environmental fluctuations such as water temperature, salinity, irradiance and nutrients. These specific characteristics make C. furca one of the dominant dinoflagellates in the coastal area
