A field experiment has been undertaken to assay the effect of Triflurotox 250 EC applied in the rates of 0, 1.5, 3.0, 4.5, 6.9, 9.0 and 12.0 mm³ kg^-1 soil on the growth and development of spring rape and white mustard and on the biochemical characteristics of the soil. The trials were conducted on leached brown soil formed from light clay silty sand of pH equal to 5.8 in 1 M KCl. The biochemical analyses were made 7 days after the experiment was established and in the flowering phase of the plants. The results proved that Triflurotox 250 EC applied in the rates varying from 1.5 to 12 mm³ kg^-1 soil had an adverse effect on the activity of dehydrogenases, acid phosphatase and alkaline phosphatase. Spring rape and white mustard were vulnerable to high concentrations of Triflurotox. The yields were negatively correlated with the herbicide rate. Potential biochemical soil activity index computed from the activities of dehydrogenases, urease, acid phosphatase and alakline phosphatase as well as the organic carbon content in the soil was negatively correlated with the Triflurotox 250 EC concentration in the soil and positively correlated with the spring rape and white mustard yields. Base saturation of soils was positively correlated with the Triflurotox concentration, in contrast to hydrolytic acidity and total exchange capacity, which were negatively correlated with the herbicide rates.