At high inorganic carbon concentration (5 to 20 mM), the photosynthetic response of Elodea densa (Planch.) Casp. to varying pH from 5 to 9 was characterized by a peak. The position of the peak shifted toward lower pH with decreasing inorganic carbon concentration. Experiments in which inorganic carbon concentration was varied at constant pH showed that the decreasing rate of photosynthesis at high concentrations was related to CO2 concentrations greater than 1 mM.