Forty-four pure cultures of marine littoral diatoms (43 pennate and 1 centric) have been studied to determine (a) whether any accessory organic factors (vitamins) are essential for growth in light, and (b) whether the cells can grow in darkness using glucose, acetate, or lactate as substrates.Six isolates were shown to require thiamine as sole growth factor; eleven require cobalamin only; and three require both thiamine and cobalamin. Sixteen are capable of heterotrophic growth using glucose but not acetate or lactate; eight can use glucose or lactate; one uses glucose or acetate; one uses glucose, acetate, or lactate; and two use only lactate.By the classical criteria of valve structure, the clones fall into about 25 different species, belonging to nine genera. However, it is evident that light microscopy is inadequate for the specific descriptions of many small diatoms. Electron micrographs of acid-cleaned frustules of each isolate served as an aid to specific identifications. Nutritional studies have demonstrated the existence of a range of distinct physiological races within the "species" Amphora coffaeiformis, Nitzschia closterium, and N. frustulum.