Six different detoxification enzymes including general esterase, permethrin hydrolase, total cytochrome P-450, aldrin epoxidase, and glutathione transferase measured with two different substrates were recorded from a variety of terrestrial and aquatic insects. Aquatic insects generally exhibited a well-developed detoxification enzyme system and often displayed activities equal to or greater than those of the terrestrial species tested. Canonical discriminant analysis revealed significant differences in overall activity profiles between terrestrial and aquatic insects. Such differences may contribute to higher sensitivity of aquatic insects to insecticide poisoning but are insufficient to explain overall differences between terrestrial and aquatic insects with regard to insecticide susceptibility.