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Four hydrolases and five oxido-reductases were examined using native stacking polyacrylamide gel electrophoresis. Homogenate of Russian wheat aphid, Diuraphis noxia (Mordvilko), bird cherry-oat aphid, Rhopalosiphum padi (L.), "Arapahoe" (aphid-susceptible) and "Halt" (aphid-resistant) wheat, Triticum aestivum L., and powdery mildew-infected Erysiphe graminis DC. ex Merat f. sp. tritici Em. Marchal, Arapahoe wheat leaves were assayed for enzyme activities. Pectinesterase, polygalacturonase (or pectinase), cellulase, and amylase activities were examined in the hydrolase group. Catalase, peroxidase, catechol oxidase, superoxide dismutase, and ascorbate oxidase activities were examined in the group of oxido-reductases. The two aphid species had the same hydrolases but different oxido-reductases. Although pectinesterase and cellulase enzymes were present in D. noxia and R. padi, the banding patterns were different. Polygalacturonase and d-amylase were not detected from either aphid species. In the oxido-reductase group, catalase was detected from D. noxia, wheras peroxidase was detected from R. padi. Superoxide dismutase and ascorbate oxidase activities also were detected from both aphids. Enzyme assays using aphid head tissue that included salivary glands but excluded aphid foregut supported the enzyme assays using whole aphids. Peroxidase activity was detected from the salivary tissue of R. padi, but not D. noxia, and catalase activity was detected from D. noxia salivary tissue, but not R. padi. We suggest that the salivary enzyme difference between the 2 aphid species (i.e., catalase and peroxidase) is important in the type of damage symptom formation on susceptible wheat plants.