Microcalorimetry was applied in a series of experiments to follow the effect caused by the pesticide 2,4-dichlorophenoxiacetic acid (2,4-D) on microbial activity of red Latosol soil. The activity of the soil was stimulated by addition of 6.0 mg of glucose and 6.0 mg of ammonium sulfate under a 35% controlled humidity in a soil sample of 1.50 mg. Power-time curves recorded on calorimeter were followed by the increasing amount of the pesticide, from 0 to 10.0 mg, which affected directly the total thermal effect evolved by microorganisms. The curves showed a synergism on total thermal effect obtained by addition of 1.0 mg of pesticide, causing a consumption of 2,4-D by microorganisms, as. new source of nutrients. However, above this mass, the total thermal effect decreased exponentially. The increase of the amount of pesticide is associated to a decrease on the microbial activity of the soil, probable due to the increase of the toxic products formed from degraded 2,4-D, affecting strongly the life in this ecosystem.