The utilization of desoxyadenylic, desoxyguanylic, and thymidylic acid phosphorus byE.colihas been studied. Under comparable conditions phosphate is utilized from the three nucleotides at about the same rate. The rate of uptake is independent of nucleotide concentration, up to about 15 μgm. of nucleotide per ml., but markedly dependent on the inorganic phosphate concentration of the medium.E.colican split off thymidylic acid phosphorus and release it into the medium as inorganic phosphate. Phosphate taken up from thymidylic and desoxyadenylic acids is distributed uniformly over the nucleotides of bacterial RNA and of DNA. It is concluded that nucleotide phosphate is assimilated exclusively as inorganic phosphorus.