Of the considerable body of literature variously concerned with the culture of algae there is relatively little work directed toward the particular problem of mass culture. Von Witsch (10, 11) grew Chlorella in vertical glass cylinders of 3 cm. diameter and studied effects of carbon dioxide provision and the composition of the medium on yield and rate of growth. He achieved yields which approached the following as maxima: 5.0 gm. dry weight per liter in 20 days; 6.68 gm. dry weight per liter in 38 days (figures from different experiments). Spoehr and co-workers (8) produced 10.69 kg. of dry Chlorella from 590 cultures grown in 5-gallon carboys under daylight illumination in a greenhouse. The cultures were harvested at 30 to 50 days ; the maximum yield achieved was about 4.0 gm. dry weight per liter. No attempt was made to investigate the factors limiting growth rate or yield. Ketchum, Lillick, and Redfield (4) studied growth and optimum yields in cultures illuminated by a jacketed neon tube immersed in eight liters of media contained in a pyrex bottle. Of a number of different unicellular algae examined Chlorella pyrenoidosa gave the highest density and daily yield, namely 0.74 gm. per liter maximum density and 0.065 gm. per liter per day maximum growth rate. If pushed to a stage of practical development, the mass culture of algae will become an engineering problem. Before any such stage is reached, however, the problem is essentially biological. The rates of growth obtained in the work cited above are lower by at least several orders of magnitude than the maximum growth rate of Chlorella of about sevenfold increase per day. The densities of population achieved are far below those obtained with other microorganisms in the fermentation industry. The biological problem is to determine the conditions necessary to obtain maximum density, rate of growth, and efficiency of light utilization. Sunlight appears to be the only feasible source of illumination for large scale mass culture. The initial experiments have been done, therefore, in a greenhouse under daylight illumination. Since the incident visible radiation per day varies from day to day such experiments must be self-contained, i.e., they must be designed to provide comparative data within any one experiment. As a starting point we chose to investigate the design of the growth chamber, particularly in regard to the most desirable thickness of the culture ex-