The culture is maintained at the optimum cell conc. for production by drawing off daily a volume of culture containing the number of cells produced during the preceding day, and replacing it with fresh sea water enriched with nutrients. The culture is illuminated by a neon tube placed in a water jacket in the axis of the cylindrical culture vessel. A 26 liter culture of Nitzschia closterium produced about 1 g. of dried diatoms daily. This quantity is nearly 3 times the average daily yield obtained when the freshly seeded culture is allowed to grow for 15 days.