During a period of phosphate starvation, the phosphate content of cells of Chlorella vulgaris which had been grown in phosphate-rich solution, decreased. The levels of most phosphate fractions declined, especially those of inorganic polyphosphates, which at first accounted for about 5 per cent of the total phosphate and virtually disappeared after 36 h starvation. On return to a phosphate medium, phosphate was taken up at a much faster rate than before starvation, with a striking increase in acid-soluble polyphosphate. The stimulated phosphate uptake and polyphosphate increase have been shown to be specific effects of phosphate starvation, occurred only when excess phosphate was supplied and required light or air for the provision of energy. There was relatively little change in the concentrations of other phosphate fractions, including orthophosphate. Inorganic polyphosphate was found to be synthesized solely from phosphate absorbed from the medium. It is argued that polyphosphate synthesis is a consequence of the stimulation of phosphate uptake, induced by the starvation period.