ABSTRACT: Karlodinium armiger is a phagotrophic dinoflagellate that synthesizes several small chloroplasts of haptophyte origin. It depends on light, but it grows very poorly in standard nutrient growth media (f/2) without food. When fed prey in the light, growth rates increase dramatically (µ=0.65d-1), suggesting that it relies heavily on phagotrophic nutrition. To explore the reasons for this phenomenon and the role of the plastids for the growth of K. armiger, we measured C14 fixation rates, chlorophyll a content and growth rates as a function of irradiance in short and long term prey starved cultures. In starved cultures, rates of photosynthesis were relatively high and increased as a function of irradiance, in spite of the fact that growth rates remained low. In prey fed cultures, both ingestion and growth rates of K. armiger increased as a function of irradiance in a saturating manner. Cellular chlorophyll a contents and photosynthetic rates were ~40% and ~70% lower, respectively, in long-term compared to short-term starved phototrophic cells at irradiances of 70-250 µmol photons m-2 s-1. Carbon fixation was important for achieving high mixotrophic growth rates, especially in prey-limited conditions. However, under prey-saturated conditions K. armiger seems to obtain most of its carbon through phagotrophy. Our data suggest that feeding stimulates the photosynthetic machinery under nutrient limitation, and that the poor phototrophic growth capacity of K. armiger in the absence of prey seems to be due to a lack or a very poor ability to take up essential inorganic nutrients.