Communal, dairy and petroleum wastewaters were separately purified in three aerobic activated sludge model reactors to investigate selected enzymatic activities of microorganisms. Four extracellular enzymes, i.e. leucine-aminopeptidase (L-AMP), beta-glucosidase (beta-GLC), alkaline phosphatase (APA) and lipase (LIP), were studied. Experimental results show that enzymatic activities associated with microbial cells in the activated sludge flocs accounted for 54.5-97.4% of the total enzyme activities in model reactors, with LIP being the most active enzyme. Highest activities of L-AMP, beta-GLC and APA were found in the dairy wastewater reactor. Kinetic parameters of the studied enzymes varied notably, high V-max followed high K-a(m), suggesting that inhibition of enzymes took place in the activated sludge systems. There were no significant relationships between enzymatic activities and dissolved organic carbon (DOC) concentrations in the three aerobic reactors during the experiments. (c) 2005 Elsevier Inc. All rights reserved.