Application to the soil of the insecticide dimethoate had no effect on the growth of soybean colonized by the arbuscular mycorrhizal (AM) fungus Glomus mosseae and by the indigenous AM fungus. The application of the recommended concentration of dimethoate decreased the percentage of colonization of soybean by the indigenous AM population, but no significant effect was observed on the colonization of soybean inoculated with G. mosseae. The insecticide did not affect the germination of G. mosseae spores; however, 0.5 mg/l of dimethoate increased the germination of Gigaspora roseae and 5 mg/l of dimethoate decreased the germination of Scutellospora castaneae spores.