Cultures of Platymonas sp., Nitzschia closterium, and Porphyridium cruentum were found to utilize the carbamino complex of alanine or sea water concentrates in preference to inorganic forms of CO2 for photosynthesis. Evidence for this was obtained by inhibition of C14O2 uptake of photosynthesizing cells upon addition of the complex. Cell growth and oxygen evolution, however, continued to increase in a normal fashion. By labeling the complex in different portions of the molecule, it was shown that the plants were utilizing the N‐carboxylate group directly as a source of CO2 while utilization of other portions was not evidenced.Several other organic compounds were tested for response in the algae but only ammonium carbonate, known to contain small portions of ammonium carbamate, acted similarly.