Green apple aphid, Aphis pomi De Geer, and Aphis spiraecola Patch (both Hemiptera: Aphididae), are sympatric aphid species that are pests of apples (Malus spp.) and other crops. A. spiraecola has been shown to be significantly more tolerant to several insecticides compared with A. pomi. To establish the mechanisms contributing to this difference in insecticide response, clones of both species were collected from British Columbia, Canada, and Washington state. Dose-response bioassays were conducted to determine relative tolerances to the insecticides pirimicarb, dimethoate, and imidacloprid; these results have been reported previously. Samples of adult aphids from each clone were assayed for the activity of esterase enzymes often involved in the detoxification of insecticides. A. spiraecola had higher esterase activity compared with A. pomi; this was apparent for two model substrates, alpha-naphthyl acetate (alpha-NA) and alpha-naphthyl butyrate (alpha-NB). Aphid clones of both species collected from Washington had higher esterase activity than clones collected from British Columbia. Clones from both species and locations hydrolyzed alpha-NA to a greater extent than alpha-NB. Esterase activity measured with both substrates was significantly positively correlated with the relative response to pirimicarb and dimethoate; a significant positive correlation also was found for hydrolysis of alpha-NB and imidacloprid. The apparent involvement of esterases in the differential response of A. pomi and A. spiraecola to insecticides indicated that the choice of control chemicals for A. spiraecola should not involve chemistries that are metabolized predominantly by esterases.