Phosphatase activities (cell-bounded phosphatases “BP” and freely dissolved phosphatases “DP”) in water samples from a natural lake “Laguna Grande” (Antequera, Málaga, Spain) amended with 50 μg/ml of selected insecticides, herbicides and fungicide captan were studied under laboratory controlled conditions (temperature and agitation). Our data show that dissolved alkaline phosphatase was the enzymatic activity that contributed in higher proportion to total lake water samples phosphatase status. The presence of organochlorinated insecticides (aldrin and lindane), organophosphorous insecticides (dimetoate, methidation and methyl-parathion), herbicide atrazine and fungicide captan significantly increased phosphatase activities after 28 days of incubation. However, these activities were not affected as a consequence of the addition of the herbicide simazine to the water samples. Heterotrophic mesophilic and psychrophilic aquatic bacteria counts as well as culturable phosphate solubilizing microorganisms, increased when the pesticides were added to lake water samples with herbicide simazine exception.