ABSTRACT: Populations of heterotrophic dinoflagellates, phytoplankton and ciliates were monitored daily during summer 1994 in the Seto Inland Sea, Japan. The growth rates of heterotrophic dinoflagellates in screened seawater (&LT50 um) were also measured. Duringthe survey period, a Gymnodinium mikimotoi red tide occurred, continued for 1 wk and disappeared. Following the outbreak of the red tide, abundance and growth rates of heterotrophic dinoflagellates (mainly composed of Gyrodinium dominans andGyrodinium spirale) increased rapidly. Calculations of the impact of grazing by the heterotrophic dinoflagellates on the G. mikimotoi red tide strongly suggest that they played an important role in the disappearance of the red tide. On theother hand, the ciliate population (mainly composed of oligotrichs of ca 45 um) did not respond to the red tide but covaried with the population of small (2 to 8 um) flagellates, suggesting that ciliates are not direct competitors of heterotrophicdinoflagellates in this region.