Polyphosphate has been isolated from the Plasmodium of the myxomycete Physarum polycephalum, and its metabolic role during growth and spherulation (differentiation) has been studied. The results of this investigation indicate that Physarum contains two types of polyphosphate distinguished by their solubilities in acid. Comparisons of both acid-soluble and acid-insoluble polyphosphate revealed the latter to be the more metabolically active form. The variability found in the amount of acid-insoluble polyphosphate appeared to be correlated with the metabolic states of the Plasmodium. In addition, changes in the amounts of other phosphorus compounds were also determined during growth and spherulation.