Cylindrotheca fusiformis Reimann & Lewin, a marine littoral pennate diatom, can grow in the dark in media containing an organic substrate. Lactate, succinate, fumarate, malate, Tryptone, Casamino Acids, or Yeast Extract can serve as substrate. Glucose apparently cannot be used for heterotrophic growth. Cells taken from the light and used as inoculum for dark cultures began to multiply after a lag period; its length depends on the substrate concentration.Experiments with radioactively labeled organic compounds confirmed the growth experiments. C. fusiformis did not take up significant amounts of 14C-labeled alanine, glutamic acid, aspartic acid, acetate, or glucose, even after several days of preincubation in the dark with high concentrations of these compounds. Cells kept in the dark in the absence of organic compounds developed a capacity to take up lactate, but not succinate. Cells grown in the dark in the presence of lactate were able subsequently to take up radioactively labeled succinate as well as lactate.The generation time under optimum conditions in the dark was around 30 h, longer than for cells growing autotrophically in the light (generation time 12.5 h at 2000 lux). The pigment composition of dark-grown cells was similar to that of light-grown cultures, except that diadinoxanthin was not detectable in the dark-grown cells. Chromatophore structure in dark-grown cells was normal. Cells brought into the light after being cultured in the dark for 3 weeks evolved oxygen immediately upon illumination. C. fusiformis did not multiply under anaerobic conditions in the dark but could survive under these conditions for at least 2 months.