The pesticide diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) was anchored onto silica gel to yield a new surface. Both free and immobilized pesticide were applied on typical Brazilian agricultural soil and the toxic effects on microbial activity of this red Latosol soil was followed by microcalorimetry. The activity of the microorganisms on 1.50 g of soil sample was stimulated by addition of 6.0 mg of glucose plus 6.0 mg of ammonium sulfate under 34.8% controlled humidity at 298.15 +/-0.02 K. The activity was recorded through power-time curves for increasing amounts of the active principle, varying from 0 to 333.33 mug g(-1). The increasing amounts of diuron, either free or immobilized, caused a decrease of the original thermal effect. The calorimetric data showed that the anchored pesticide presented a much lower toxic effect than free diuron on microbial activity.