Prey selectivity, growth and feeding responses were studied in the ciliate Balanion comatum (17 μm) and the heterotrophic dinoflagellate Gymnodinium sp. (7 μm). Almost identical prey size spectra were found for the 2 organisms. Optimum prey size was 8 μm, while the lower and upper limits of prey capture were ~4 and 10 μm, respectively. Maximum growth and ingestion rates of B. comatum were slightly higher than those of Gymnodinium sp. Threshold prey concentration for growth of B. comatum and Gymnodinium sp. was 11 and 17 μg Cl ^-1 , respectively. At 15°C, both organisms needed to ingest approx. 1 to 2% h ^-1 of their cell volume in order to sustain basic metabolic activity. Maximum specific clearance was 2 to 3 times higher for the ciliate compared to the dinoflagellate. Gymnodinium sp. survived for a longer time than B. comatum when deprived of prey organisms. Gymnodinium sp. cells were not ingested by B. comatum, although they were of a size which is optimal for B. comatum.