Navicula pavillardi Hustedt, a marine, littoral, pennate diatom, can grow in the dark on glutamate or on the complex organic supplements tryptone or yeast extract. Growth on glutamate in the dark took place without an initial lag phase, whereas growth on tryptone began only after a 2-day lag phase that could be abolished by the simultaneous presence of glucose. Lactate inhibited growth in the dark on glutamate, but not photoautotrophic growth. Relatively low concentrations of glutamine inhibited photoautotrophic growth. The observed doubling time for heterotrophic growth on glutamate or tryptone was about 70 h, compared with a doubling time of 24 h under optimal photoautotrophic conditions. Glucose did not decrease the doubling time in the dark on tryptone. The assimilation efficiency for glutamate was 41%. The estimated necessary uptake rate for glutamate to account for the observed heterotrophic doubling time on glutamate was close to those measured with isotope techniques. The kinetic parameters for glutamate uptake, which followed Michelis-Menten kinetics, were Ks = 0.018 mM, and Vmax = 7.0 X 10(-10) mumol per cell per minute. Although several amino acids served as sole nitrogen sources for photoautotrophic growth and were demonstrated by the use of isotope techniques to enter the cells, they could not be used as substrates for growth in the dark. Glucose was not taken up to a significant extent except by cells grown in the presence of tryptone. Lactate was taken up only by dark-grown cells. Results of preliminary studies on the metabolic fate of several uniformly labeled amino acids are presented.