Assimilation of nitrogen by nitrogen-deficient Chlorella cells was investigated. Assimilation occurred very actively at first in the dark, and almost entirely stopped within a few hours, during which remarkable decreases in free reducing sugar, sucrose and water-soluble polysaccharide in the cells were observed. Most of the nitrogen assimilated was stored as soluble nitrogen. Assimilation of nitrogen was accom-panied with the activation of respiration and photosynthesis. Activation of respiration ceased when the nitrogen was completely absorbed, but that of photosynthesis lasted thereafter. The latter resulted in stimulation of growth, and favoured very much the operation of semi-continuous and continuous cultivations of nitrogen-deficient fatty cells. It was as-certained that these cultivations can be continued for a certain length of time with much better yield of fat than expected owing to the activation of growth, though the fat content of cells shows a slightly decreasing trend in the course of the operation in the state of equilibrium. The stimulation of photosynthesis was considered to be due to the activation of the enzymes or the enzyme systems involved in the carbon dioxide fixation mechanism.