A dimethoate-degrading enzyme from Aspergillus niger ZHY256 was purified to homogeneity with a specific activity of 227.6 U/mg of protein. The molecular mass of the purified enzyme was estimated to be 66 kDa by gel filtration and 67 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric point was found to be 5.4, and the enzyme activity was optimal at 50℃ and pH 7.0. The activity was inhibited by most of the metal ions and reagents, while it was induced by Cu21. The Michaelis constant (Km) and Vmax for dimethoate were 1.25 mM and 292 mmol min21 mg of protein21, respectively.