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Strains of German cockroach, Blattella germanica (L.), were tested for resistance to three insecticides using the time-mortality response technique in comparison with known susceptible strains. Dursban-R and Macy strains indicated high levels of resistance to chlorpyrifos and moderate resistance to propoxur and cypermethrin. The hydrolytic activity of esterase enzymes was determined with a model substrate, p-nitrophenyl acetate. The specific activity of the enzymes increased with a corresponding increase in substrate concentration. Maximum activity was observed at 80 μM. The resistant strains had Significantly higher activity than the susceptible strains. The enzyme activity also increased with an increase in pH. Highest esterase activity was observed at pH 8.0 for all the strains. The calculated Michaelis Menten constant (Km) values for the resistant strains were nearly two-fold higher than for the susceptible strains. Nondenaturing gel electrophoresis revealed differences in isozyme composition. In total, 10 esterase bands were detected, and these bands were characterized using different inhibitors and substrates.