SUMMARY: Forty-two bacteria-free cultures of pennate diatoms from fresh water or soil were tested for ability to grow heterotrophically with glucose as the sole carbon source. Thirteen of these isolates proved capable of growth under these conditions. They comprised seven isolates of NawicUlu pelliculosa, five of other Merent species of Nawieula, and one of Nitzschiu (3) fontiCola. One isolate of NavicUla pellieubsa was tested for its ability to grow hetero- trophically on any one of sixty organic compounds. Only glucose, glycerol, and fructose were found to be effective in supporting growth. Glucose was metabolized both in the dark, and in the light in the absence of CO,. Glycerol and fructose were not utilized in the dark, but supported growth in the light in the absence of CO,. There is apparently no previous record that any chlorophyllous diatom has been cultivated successfully in the dark. Miquel (1892) found that certain species of diatoms which he obtained in pure culture would not develop either in the dark or in semi-darkness on organic nutrient solutions, although they retained the power of proliferation when returned to the light after several months in darkness. Benecke (1900) observed no increase in numbers in contaminated cultures of pigmented diatoms maintained in the dark, either in hanging drop cultures or in mass cultures with a rich organic nutrient. Karsten (1901), working with impure diatom cultures, determined growth by making daily cell counts. After the cells had been placed in the dark on a medium containing glycine and glucose, he observed three cells of NuviMlla perpzctPiUa to increase to nineteen in 6 days, and seven cells of Nitzschiu cbstedurn to increase to eighteen in 3 days. Later workers have been inclined to discredit Karsten's observations because he worked with impure cultures, and because he did not let his experiments run for a long enough period to establish conclusively that the organisms were capable of using an external substrate for growth. Karsten also reported that Nitzschia palea, unlike the diatoms previously mentioned, showed only an occasional cell division in dark- ness. He observed that eleven cells increased to fourteen cells in 2 days, and no further growth was noted after 10 days. Richter (1906) reported that pure cultures of Nuviczlla minzlsczlla showed no significant development after a month in the dark on a mixture of glycerol, asparagine and glucose, but considered that slight growth took place in his pure cultures of N. palea during the first day in darkness, in agreement with Karsten's observations on the same organism. Meinhold (1911) and Treboux (1905) were unsuccessful in obtaining growth of diatoms in the dark. Although he did not specifically attempt to obtain growth of N. palea in the dark, von Denffer (1947) observed that one cell division could occur in darkness when the cells were well filled with fatty reserves. Such cells were able to