Cultures of Cblorelka vulgaris reach the stationary phase of their rowth cycle more slow1 than cultures of C. &reno&sa when both are provided w i z conditions favorable &r their growth. But after 3 weeks of incubauon, 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 mlgaris. There is a somewhat larger difference in lipid content, C. &renoidosa yieldin about 1 1 to 12 per cent more lipid than C. vulgaris. Both species produce more lipit and relatively less rotein with increasing age. Accumulation of lipid in excess of about 2 5 per cent o&otal dry weight is indicative of waning vigor in cultures of Cblorella, and a lipid content that equals or exceeds the protein content is indicative of extreme senescence. Both organisms grow oorly in a nitro endeficient medium but M e r in their response to it. The yield z o m C. vnl aris is a L u t 80 to 86 per cent of the yield from C. pu*enoidosa with respect to packefvolume, d r y 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