ABSTRACT: To investigate the interactions between the ciliate Strombidinopsis jeokjo and the heterotrophic dinoflagellates Gyrodinium dominans and Oxyrrhis marina, we measured the growth and ingestion rates of S. jeokjo (mean length and width of fixed specimens = 149 × 70 μm, n = 30) when feeding on each of the heterotrophic dinoflagellates, and calculated grazing coefficients by combining field data on abundances of large Strombidinopsis spp. (>100 μm in length) and co-occurring heterotrophic dinoflagellates with laboratory data on ingestion rates obtained in the present study. The specific growth rates of S. jeokjo when feeding on G. dominans and O. marina increased rapidly with increasing prey concentration (ca. <300 ng C ml^-1), but were saturated or slightly increasing at higher concentrations. The maximum specific growth rate of S. jeokjo feeding on G. dominans (0.54 d^-1) was similar to that when fed on O. marina (0.59 d^-1). Threshold prey concentrations (where net growth = 0) were 79 ng C ml^-1 for G. dominans and 36 ng C ml^-1 for O. marina. The maximum ingestion and clearance rates of S. jeokjo feeding on G. dominans (108 ng C grazer^-1 d^-1 and 14.5 μl grazer^-1h^-1, respectively) were comparable to those when fed on O. marina (87 ng C grazer^-1 d^-1 and 13.4 μl grazer^-1 h^-1, respectively). Calculated grazing coefficients for large Strombidinopsis spp. feeding on G. dominans and O. marina were 0.01 to 0.39 h^-1 (i.e. 1 to 33% of G. dominans populations were removed by Strombidinopsis spp. populations in 1 h) and 0.002 to 0.004 h^-1 (i.e. 0.2 to 0.4% of O. marina populations were removed), respectively. The results of the present study suggest that Strombidinopsis spp. can sometimes have a considerable grazing impact on populations of G. dominans and O. marina.