Seasonal variation of kinetics parameters of total alkaline phosphatase in the eutrophic zone of a shallow Chinese freshwater lake (Donghu Lake) was reported.The concentration of chlorophyll and V_max of total alkaline phosphatase reached the highest level when ambient orthophosphate concentration was low and the values of Michaelis constant(K_m)of alkaline phosphatase decreased.Thus, phytoplankton seems to compensate for their phosphorus deficiency not only by an increase in enzyme activity but also an improved ability to use low substrate concentrations. The k_m of alkaline phosphatase bore a direct positive relation to UV-sensitive P compounds (UVSP). A better relationship by plotting him values against the ratio UV SP:phosphmmonoester is also found, suggesting that UVSP is a competitive inhibitor of alkaline phosphatase activii.yin the lake.