Toxic effects of five commonly used pesticides on the biomass of a municipal activated sludge system were determined on the basis of the reduction in the oxygen uptake rate (OUR) and specific oxygen uptake rate (SOUR). Toxicity levels of the selected pesticides were determined by employing a modified OECD 209 (Organisation for Economic Cooperation and Development) method which was performed as batch experiments using a respirometer. Copper sulphate (CuSO4. 5 H2O), copper oxychloride (Cu2Cl(OH)(3)), copper calcium oxychloride (CaCu3Cl2(OH)(6)) as copper-based pesticides and chlorsulphuron (C12H12ClN5O4S), 2,4-dichlorophenoxyacetic acid (2,4-D) (C8H6Cl2O3) as synthetic organic pesticides were selected for the experiments. The EC50 values were determined to be 78, 249 and 281mg/L for CuSO4.5 H2O, Cu2Cl(OH)(3) and CaCu3Cl2(OH)(6), respectively. Corresponding values for C12H12ClN5O4S and 2,4-D were 860 and 3664mg/L, respectively. Results indicated that toxicity effects of copper-based pesticides were higher than that of synthetic organic pesticides. CuSO4.5 H2O was found to exert the highest toxicity among the copper-based pesticides, whereas, C12H12ClN5O4S was determined to be the most toxic among the organic pesticides on activated sludge biomass.