The effect of mesotrione on microbiological activity in soil was investigated. Trials were set up in laboratory on chernozem soil (pH 7.0, organic matter 3.5%, sand 26%, silt 45%, clay 29%) at Surcin, Serbia. Mesotrione was added at rates 0.5 (field rate), 5, 25 i 50 mg/kg soil. Untreated soil served as control. Samples were collected for analysis 5, 20, 40 and 60 days after mesotrione application. The effects were assessed on bacteria abundance, fungi abundance, and dehydrogenase activity. Mesotrione was found to cause different effects on the soil microbial activity in soil and its influence depended on the rate of application and duration of activity. Mesotrione applied at 0.5 and 5 mg/kg soil did not have any effect on microbial activity. The higher herbicide doses (25 and 50 mg/kg) induced increasing activity from the 5th to 60th day. These experimental data indicated that mesotrione affected soil microbial activity, but the effects were only detected at higher doses far exceeding the recommended field rate.