The composition and activity of alkaline phosphatase in waters with dif ferent PO₄^3--P concentrations were studied ex perimentally.The results show:①The total alkaline phosphatase activity (TAPA)was decreased with the increase of PO₄^3--P concentration (r=-0.586,n=55). Based on the statistic analyses, when PO₄^3--P concentrations were between 0 to 0.010mg·L^-1, no significant dif ference was found in the TAPA.When PO₄^3--P concentrations exceeded 0.010mg·L^-1, the TAPA was significant difference (P<0.01).②The contents of APA in phy toplankton (PAPA), bacteria (BAPA)and dissolved (DAPA)were different.The ratio of DAPA to TAPA was the highest and BAPA was the smallest.③With the increasing of PO₄^3--P concent ration, the PAPA decreased.When PO₄^3--P concent rations were between 0 to 0.020mg· L^-1, over 0.020mg·L^-1 and over 0.200mg·L^-1, the PAPA were no significant, significant (P<0.05)and highly significant, respectively (P<0.01).④The BAPA showed no signif icant relationship with the PO₄^3--P concent ration.