Cultures of Chlorella vulgaris reach the stationary phase of their growth cycle more slowly than cultures of C. pyrenoidosa when both are provided with conditions favorable for their growth. But after 3 weeks of incubation, the yields from the two organisms are nearly identical in terms of packed volume, dry weight, and cell population. There is only a relatively small difference in protein content—about 8 per cent in favor of C vulgaris. There is a somewhat larger difference in lipid content, C. pyrenoidosa yielding about 11 to 12 per cent more lipid than C. vulgaris. Both species produce more lipid and relatively less protein with increasing age. Accumulation of lipid in excess of about 25 per cent of total dry weight is indicative of waning vigor in cultures of Chlorella, and a lipid content that equals or exceeds the protein content is indicative of extreme senescence. Both organisms grow poorly in a nitrogen-deficient medium but differ in their response to it. The yield from C. vulgaris is about 80 to 86 per cent of the yield from C. pyrenoidosa with respect to packed volume, dry weight, and per cent lipid. Only with respect to per cent protein are the two organisms the same. However, in absolute terms (mg./ml. of culture) C. vulgaris yields about 14 per cent less protein and nearly 30 per cent less lipid than C. pyrenoidosa in such a medium.