1. 1. The mycelium of A. niger, growing in stationary and shake cultures in synthetic Currie's medium, with and without zinc supplementation, has been fractionated for its phosphate constituents, with special reference to “soluble” and “insoluble” metaphosphates, RNA and DNA. 2. 2. The non-nucleic acid phosphate of the mycelial samples has been identified as (insoluble) metaphosphate by the labile nature of the phosphate toward hot dilute acids, barium precipitation at acid pH, positive metachromasy, and paper chromatographic analysis. Quantitative assay of metaphosphate by the metachromatic reaction indicates that almost the entire non-nucleic acid phosphate in the mycelial samples tested consists of highly polymerized metaphosphate. 3. 3. Metaphosphate is present in all mycelial samples in both the “soluble” and the “insoluble” forms. 4. 4. Soluble metaphosphate is present in low concentrations to begin with, but accumulates as the organism grows and utilizes sugar and mineral salts. A peak is reached, after which the metaphosphate concentration falls, to be followed by a sharp rise at the time of general autolysis. In the absence of added zinc in the medium, there is a higher accumulation of metaphosphate. 5. 5. Insoluble metaphosphate is present in very high concentration in the initially formed mycelium. In surface cultures this high level is maintained for about a week, after which there is a steady fall. In rotary cultures, on the other hand, the high level of metaphosphate is maintained throughout the incubation period. 6. 6. The total amount of metaphosphate (soluble and insoluble) is high in the cultures where mycelial growth has been restricted, and low where the mycelial growth is good. 7. 7. The initially formed mycelium contains a high concentration of nucleic acids. This level is maintained for the first few days, but falls steadily with progressive incubation. RNA predominates initially, but rapidly disappears so that toward the end of the incubation period the ratio of DNA to RNA is considerably increased. The rapid diminution in the RNA has no parallel in the other phosphorylated compounds. 8. 8. It is suggested that soluble metaphosphate functions as a comparatively inert phosphagen, gradually accumulating and disappearing. Insoluble metaphosphate is probably an active phosphagen playing an essential role, in conjunction with RNA, in the early stages when the mycelium passes through a phase of intense metabolic activity. Soluble metaphosphate is probably derived from the insoluble metaphosphate by the loss of phosphate residues.