The application (up to 5 kg ha-1) of monocrotophos or quinalphos to four types of agricultural soils significantly stimulated the mineralization of peptone-nitrogen and the oxidation of ammonium-nitrogen. In soils treated with 2.5 kg ha-1 of either insecticide, the rate of ammonification and nitrification was fairly rapid after 2 and 4 weeks of incubation. The enhancement of both transformations, mediated by microorganisms, was significantly more in the quinalphostreated soils. The results suggest that a balance between the effect of insecticides on insect pests and their impact on beneficial microbial activities in soil must be determined.