SUMMARYThree pelagic marine phytoplankters, Coccolithus huxleyi, Skeletonema costatum,andThalassiosira rotula,and a facultative heterotroph, Cyclotella cryptica,have been exposed to three organic substrates, viz,glucose, acetate, and glutamate, at low concentrations (organic carbon 0.25 mg/liter). Experiments were performed in the dark and light and the net assimilation of substrate was measured by using radiocarbon. The dark uptake of carbon dioxide was also determined, together with photosynthesis at near optimum light intensity. The expected heterotrophy was detected withCyclotella cryptica. Thalassiosira rotulawas found to assimilate glutamate at an appreciable rate. In all cases, however, the short‐term uptake of carbon dioxide in the dark was the greatest assimilation rate measured. Values are discussed in relation to their ecological significance and it is concluded that heterotrophic survival of these and probably most other algae in the open ocean would be impossible unless they were in contact with a high concentration of substrate in the form of particulate matte