AbstractA study of the effects of the insecticide chlorpyrifos on soil microbial activity was made. Parameters included total bacterial populations, fungi, aerobic N2-fixing bacteria, denitrifying bacteria, nitrifying bacteria, nitrogenase activity, acid and alkaline phosphatases, arylsulfatase, and dehydrogenase. The presence of 2.0, 3.5, 5.0, and 10.0 kg/ha of chlorpyrifos significantly (p < 0.05) decreased aerobic dinitrogen-fixing bacteria and dinitrogen fixation, particularly after a second insecticide treatment The total number of bacteria increased significantly at concentrations of 2.0 to 10.0 kg/ha. Activities of acid and alkaline phosphatases and dehydrogenase significantly decreased initially at concentrations of 2.0 to 10.0 kg/ha, but recovered after 14 d to levels similar to those in control soil without chlorpyrifos. Fungal populations, nitrifying bacteria, and denitrifying bacteria were not affected as a consequence of the addition of the insecticide.