N. closterium, N. bilobata, Navicula sp. and Chlorella sp. multiply more rapidly in sea-water media to which ammonium salts have been added than in the presence of nitrates. The optimum concn. of ammonium N is 0.01-0.05 millimols; the nitrate optimum is about a hundred times greater. The addition of a little ammonium to nitrate sea-water medium stimulates growth. In media containing both nitrates and ammonium, chemical analyses reveal that the ammonium is assimilated first. Nitrates are reduced to nitrites by the phytoplankton in darkness as well as in the light. Nitrites are assimilated. It is believed that in the sea ammonium salts play a very important role in the nitrogen nutrition of phytoplankton.