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Esterase activity between the resistant Crawford and susceptible CSMA strains of German cockroach, Blattella germanica (L.), was compared with the substrates α- and β-naphthyl acetate across sex and nymphal age classes. Esterase isozyme analysis with native polyacrylamide gel electrophoresis also was conducted to identify quantitative and qualitative differences between strains, sexes, and age classes. The Crawford strain was highly resistant to cypermethrin, propoxur, and permethrin with a resistant ratio (RR) of 17.26,15.75,and 13.53, respectively, and mildly resistant to chlorpyrifos (RR 5.62). The α-NA and β-NA esterase activities in the Crawford strain were significantly higher than those activities in the CSMA strain in both nymphal and adult stages. In the Crawford strain, the enzyme activity in nymphs was significantly higher than that in adults, but such differences were not observed in the CSMA strain. The mobile isozymes a and c stained more intensely than others in every developmental stage and sex of both strains but showed greater intensity in the Crawford strain. Another intensely stained isozyme b was observed only in the homogenates from the Crawford strain. The combination of isozyme b and the overproduced isozyme II and c in the Crawford strain seems to be responsible for the difference in total esterase activity between the CSMA and Crawford strains.