ABSTRACT: The functional and numerical responses for the marine obligate mixotrophic ciliate Mesodinium rubrum Lohmann, 1908 (=Myrionecta rubra Jankowsky, 1976) were studied at 2 irradiances (20 and 100 µE m² s^–1). Furthermore, its tolerance to high pH levels and response to starvation were studied in mixed cultures of M. rubrum and Teleaulax sp. The functional and numerical response study showed that the threshold concentration of the cryptophyte Teleaulax sp. was 50 cells ml^–1 and the maximum growth of M. rubrum was 0.23 and 0.49 d^–1 for 20 and 100 µE m² s^–1, respectively. Calculation of ingestion rates revealed that ~1 Teleaulax sp. cell M. rubrum^–1 d^–1 was sufficient to maintain the maximum growth rate. Maximum ingestion rates were independent of light and saturated at ~6 Teleaulax sp. cells M. rubrum^–1 d^–1. A heterotrophic carbon uptake of from 2 to 4% of M. rubrum carbon content was sufficient for maximum growth, but carbon contributions as high as 22% were observed to have no effect on growth. The pH experiments revealed that the growth of M. rubrum and Teleaulax sp. was impeded at pH levels in excess of 8.5 and 8.8, respectively. Experiments to reveal M. rubrum’s response to starvation showed that M. rubrum could survive for around 50 d without prey. These results are all discussed with respect to M. rubrum’s adaptation to its environment.