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A susceptible and two parathion-resistant strains of the greenbug, Schizaphis graminum (Rondani), exhibit three different patterns of general esterase isozymes in native polyacrylamide electrophoresis gels. Characterization of general esterase activity using α -naphtholic esters as model substrates indicated that the three strains differed in isozyme composition. The type-I1 strain, which had the highest level of resistance, exhibited the highest levels of general esterase activity under all assay conditions, and the type-I strain had consistently higher levels than the susceptible strain. In all three strains, these esterases were more active toward α -naphtholic esters with side chains of six or fewer carbon atoms. α -Naphthyl propionate was the optimal substrate for both susceptible and type-I strains, and α -naphthyl butyrate for type 11. Over 90% of esterase activity was localized in the cytosolic fraction of type-I1 greenbugs. In susceptible and type-I greenbugs, the activity was distributed equally between the cytosolic and microsomal fractions. Differences in kinetic properties of the general esterases from the three strains also were evident, further indicating differences in isozyme composition. Although the three strains differed in properties of the general esterase activities, the differences do not provide sufficient discrimination to distinguish reliably among the three strains using single aphid activity measurements.