The efficiency of conversion of light into organic matter by cultures of growing Chlorella cells was measured under various conditions. During growth periods of 24—180 hours, we daily measured the incident light intensity, the fractional absorption, and the photosynthetic and respiratory rates. Finally, the increase in dry weight and the chemical composition of the harvested algae were determined. A correlation between chemical composition and heat of combustion was established. In this way a complete balance of the energy transformations during algal growth was obtained. _ . , calories fixed in organic matter , _ _ Efficiencies were expressed as ;—:— ;—= X 100. calories light absorbed Optimal efficiencies of 20—24 % for conversion of sodium light, representing about the mean of the visible solar radiation spectrum, were found in culture media, containing nitrate. Under conditions of nitrogen starvation which leads to cells of low protein and high carbohydrate and fat content, the yield diopped rather quickly. Strong continuous illumination induced cells with low nitrogencontent, whereas cycles of light and darkness favoured the occurence of algae rich in protein. A relatively small decrease in overall efficiency is caused by natural day and night periods. The linear relation found between growth and intensity and time of exposure, which means that the yield is independent of cellular concentration, is discussed.