Mixotrophy in Prymnesium parvum was investigated using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes. The experiment was performed in light and dark. In the dark treatment we expected that the mixotrophic P. parvum would rely solely on its prey and therefore reflect the prey isotopic signatures. In the light treatment P. parvum can perform photosynthesis as well as utilize its prey, thus we expect the isotopic signatures to be between the dark mixed cultures and the monocultures, depending on how much prey was utilized. In the light treatment, addition of the ciliate Myrionecta rubra resulted in higher P. parvum cell numbers compared to monocultures. During the experiment, cell numbers in the dark monocultures and the mixed dark cultures did not increase. P. parvum had 2.5-3 times higher cellular phosphorus and nitrogen content in the dark compared to the cultures in the light whereas no difference in carbon content between treatments could be observed. This suggests that P. parvum can utilize nitrogen and phosphorus but not carbon in the dark. It thus seems as if P. parvum relies on photosynthesis to meet the carbon and energy demand required for growth. The expected isotopic signatures “become what you eat…plus a few per mil” were not observed. In the dark treatment, the δ¹³C did not differ between monocultures and mixed cultures. In the light treatments P. parvum δ¹³C became less negative then the corresponding dark treatments indicating that P. parvum used CO₂ rather than carbon from the added prey. No difference in δ¹⁵N between monocultures and mixed cultures could be observed during the experiment. We argue that light is necessary for P. parvum growth and that the ability to utilize nutrients originating from their prey may be important in bloom formation.