Dinophysis acuminata is a mixotrophic dinoflagellate frequently causing diarrhetic shellfish poisoning. D. acuminata was isolated from Danish coastal waters and cultivated using the mixotrophic ciliate Mesodinium rubrum as prey. The roles of food uptake and photosynthesis for the growth and survival of D. acuminata were studied. The observed maximum growth rate was ca. 0.45 d^-1 at an irradiance of 100 µmol photons m^-2 s^-1 when supplied with well-fed M. rubrum at concentrations >1000 M. rubrum ml^-1. The corresponding ingestion rate per D. acuminata was about 9 M. rubrum d^-1, or 3200 pg C d^-1. Apart from the importance of prey concentration, data revealed that ingestion and growth of D. acuminata are, to a certain degree, dependent on the growth rate of its ciliate prey. Photosynthesis was studied in a culture of D. acuminata, initially grown at prey saturation (>1000 M. rubrum cells ml^-1) and subsequently allowed to deplete the prey. When the ciliate prey was added, the photosynthetic rate of D. acuminata increased from about 7 to 38 pg C cell^-1 h^-1 within the first 2 d. When subsequently subjected to starvation, the photosynthetic activity rapidly decreased to a pre-feeding level, and, without a minimum food uptake, D. acuminata was not able to maintain photosynthetic growth. At high prey concentrations, 70 to 90% of the gross C uptake can be explained by food uptake. However, at low prey concentrations, only 0 to 55% of the gross C uptake is likely to be derived from food uptake. The data indicate that in natural environments, D. acuminata may often be food limited, and, in this situation, photosynthesis may not only be a supplement to the basic nutrition, but rather the primary C source.