The rates of phosphate uptake of young intact plants were measured in solution at concentrations between 3.2×10−7 M and 3.2×10−5 M KH2PO4. The addition of 0.005 to 0.01M CaCl2 increased the uptake rates to values of up to 4 times those of controls. The increases in uptake relative to the controls were most marked at the lowest phosphate concentrations. The addition of KCl also increased the uptake rates, but to a much smaller extent. The results are analyzed by the enzyme kinetic theory, and it is concluded that the uptake of phosphate at low ionic strengths is impeded by negative potentials at the root surface.