The normal level of phosphorus in Anacystis nidulans is approximately 3.7 μg Pi/mm3 cells. This value fell to 0.5 μg Pi/mm3 cells under prolonged starvation. Even at low cellular phosphate levels, cells were viable and continued to divide slowly. With cells containing approximately 1.5 μg Pi/mm3 cells a rapid dark uptake (15 minutes) of 0.8 μg Pi/mm3 cells was found. Data obtained in the rapid dark fixation suggest that approximately 25% of the total cellular phosphorus is possibly bound on specific sites. Light had little effect on this first phase of phosphate uptake. The subsequent uptake to the normal phosphorus content per cell and return to normal growth rate required light and nitrogen.Coincident with the rapid dark phosphate incorporation, synthesis of ATP began and continued, rising far above the level of normal cells. The rate of ATP formation was not influenced by light, but was blocked by anaerobic conditions or several classical inhibitors of the electron transport chain.