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Red tides dominated by the harmful dinoflagellate Cochlodinium polykrikoides have caused annual losses of USD $5–60 million to the Korean aquaculture industry annually since 1995 and a loss of USD $3 million during a 1999 net-pen fish mortality event in Canada. In order to evaluate the potential to control C . polykrikoides red tides dominated by using mass-cultured heterotrophic protistan grazers, we monitored the abundance of Strombidinopsis jeokjo (a naked ciliate) and C . polykrikoides after mass-cultured S. jeokjo was introduced into mesocosms (ca. 60 l) deployed in situ and containing natural red tide waters dominated by C . polykrikoides . Water temperature, salinity, and pH, as well as the abundance of co-occurring other protists and metazooplankton were measured concurrently. To compare the growth and ingestion rates of S. jeokjo feeding on cultured versus natural populations of C . polykrikoides , we also monitored the abundance of cultured C . polykrikoides and S. jeokjo in bottles during laboratory grazing experiments. S. jeokjo introduced into the mesocosms grew well, effectively reducing natural populations of C. polykrikoides from approximately 1000 cells ml −1 to below 10 cells ml −1 within 2 days. The growth and ingestion rates of cultured S. jeokjo on natural populations of C. polykrikoides in the mesocosms for the first 30 h (0.72 day −1 and 51 ng C grazer −1 day −1 ) were 84% and 44%, respectively, of those measured in the laboratory during bottle incubations with similar initial prey concentrations. The calculated grazing impact of S. jeokjo on natural populations of C. polykrikoides suggests that large-scale cultures of this ciliate could be used for controlling red tides by C. polykrikoides in small areas.