The effect of irradiance on growth and grazing responses of 2 phagotrophic dinoflagellates, Gymnodinium gracilentum Campbell 1973 and Amphidinium poecilochroum Larsen 1985, was studied. While G. gracilentum belongs to the plankton, A. poecilochroum is a benthic species that primarily feeds on prey associated with surfaces. Both organisms are able to retain functional chloroplasts from their prey. They are both able to grow heterotrophically in the dark, but growth rates increase in the light. The maximum growth and ingestion rates of G, gracilentum are much higher than those of A. poecilochroum, However, the growth rate of A. poecilochroum is saturated at a lower irradiance (similar to 6 mu mol photons m(-2) s(-1)) than to G. gracilentum (similar to 60 to 80 mu mol photons m(-2) s(-1)). Also, the irradiance required for saturation of growth for both dinoflagellates matched that found for the prey algae. The effect of light on ingestion and growth was also studied during the light and dark periods of the day. Ingestion rates of G. gracilentum were higher during the light period, while division rates were higher during the dark period. Offered a variety of prey items belonging to different algal classes, G, gracilentum selectively feeds on species belonging to the class Cryptophyceae.