Phosphatase activity was measured with other environmental factors in Cheonho reservoir in 1994. It ranged form 95 to 1,685 nM/1/h and was correlated significantly with chlorophyll-a. Such a close relation well matched the fact that over 90% of phosphatase activity was detected in > 3 $mu extrm{m}$ fraction. The phosphatase activity also correlated negatively with dissolved inorganic phosphate concentration, which implies derepression of phosphatase production by phosphate limitation. Significant correlation was analyzed between phosphatase activity and BOD, which also appeared to be closely correlated with chlorophyll-a. A great percentage of organic materials seems to be generated autochthonously by algae and extracellular enzyme even though allochthonous influence was thought to be stronger in Cheonho reservoir.