Biochemical decomposition of the water solution of the herbicide Ethofumesate (EFS) with the actived sludge (AS) adapted on EFS was studied. The EFS initial concentration was in the range S (0) = 0.05 - 0.2 mmol dm -3, the AS initial concentration (expressed as the weight of dry biomass in the volume unit of the reaction mixture) was X(0) = 0.4 - 2.2 g dm -3. Experiments were carried out in a batch isothermal stirred reactor at temperatures of 15 - 40 °C. It was found out that EFS is decomposed by means of AS very slowly. At given conditions, the initial velocity of the EFS docomposition was v = 3 to 30 mg EFS h -1 g -1 AS. Concentrations of AS and dissolved oxygen in the reaction mixture were practically constant during the whole reaction time in all the experiments. The value of v decreased at 20 - 25 °C with increasing ratio of initial AS and EFS concentrations p = X0/S0. At temperatures of 30 and 35 °C, an inductive period of ca two hours took place before EFS decomposition. In the liquid phase of the reaction mixture an unknown substance was found beside EFS, which is not a decomposition product of EFS bur originates probably from the microorganisms of AS. All the analyses carried out indicate a complicated reaction mechanism (probably dictated by physical rather then chemical steps) and the fact that EFS is a species hard to decompose biologically.