Competition for phosphate was studied in Scenedesmus sp. and Pseudomonas sp. grown in pure and mixed cultures. Algal growth was severely limited in the presence of bacteria, but the growth of bacteria was hardly affected by algae. The first sign of competition in mixed cultures (cessation of exponential algal growth) was always observed some time after the external phosphate had been exhausted.The faster growth rate of bacteria accounted for the suppressed growth of the algae in the mixed cultures. The first sign of competition in algae appeared when the concentration of surplus phosphorus within the algal cell dropped to a critical value near zero. The surplus phosphorus appeared to consist of three or all four inorganic polyphosphate fractions.