The phosphate requirement, i.e. the concentration of inorganic orthophosphate that just ceases to be limiting for growth, of Escherichia coli N.C.T.C. 5928 was determined for growth in ammonium-salts media containing glucose or acetate as the carbon and energy source, and compared with that of six other strains of E. coli. 2. The phosphate requirement for E. coli N.C.T.C. 5928 growing on acetate was about ten times that for growth on glucose, but this difference was not observed with any of the other strains. 3. After about 40 generations' growth on acetate with phosphate limitation in a chemostat, the phosphate requirement of the cells gradually decreased until it was equivalent to that of the glucose-grown organism; a single passage through glucose batch culture sufficed to restore the original high phosphate requirement, indicating a permeability phenomenon. 4. The lipid content of E. coli N.C.T.C. 5928 grown on glucose or acetate was measured isotopically by fractionation of cells grown on inorganic [(32)P]orthophosphate and gravimetrically after extraction from the cells by three different methods; change of carbon source from glucose to acetate did not affect the lipid content, which remained constant at 8-9% of the bacterial dry weight.
