Chattonella antiqua and C. marina are known to be the most noxious red tide flagellates causing mass mortality of cultured fish. Their optimum environmental requirements for growth and survival ranges, such as temperature, salinity and irradiance, were determined with axenic cultures. The growth of C. antiqua and C. marina were observed at the irradiance of 30μE/m²/s or more and saturated at 110μE/m²/s. The growth responses of the flagellates were examined at 30 combinations of different temperatures (10-30°C) and salinities (10-35‰) under saturated light intensity of 120μE/m²/s. They grew at temperatures from 15 to 30°C and salinity from 10 to 35‰. Maximal growth rates of 0.97 divisions/day (C. antiqua) and 0.81 divisions/day (C. marina) were obtained with combinations of 25°C and 25‰ and 25°C and 20‰, respectively. At suboptimal temperatures, both species had reduced salinity-tolerance ranges. A statistical test indicated that temperature had the greatest influence on growth rates, followed by salinity and their interaction. The present study reveals that C. antiqua and C. marina are eurythermal and euryhaline organisms. These physiological characteristics presumably allow them to develop huge blooms during the summer in the Inland Sea of Japan.