Resistance of diamondback moth (DBM), Plutella xylostella (L.) to insecticides applied for its control on cabbage was evaluated. DBM populations were tested for susceptibility to three pyrethroids (delatamethrin, lambdacyhalothrin, cypermethrin) and an organophosphate (chlorpyrifos-methyl) insecticide using larvae immersion dose-response technique. Non-specific carboxylesterase activity was used as a measure of DBM resistance. Partial characterization of carboxylesterase by zymogram studies was also undertaken using polyacrylamide gel electrophoresis (PAGE). Susceptibility studies revealed that DBM was resistant to the four insecticides assayed. DBM was least susceptible to chlorpyrifos-methyl, but most susceptible to deltamethrin. Esterase zymogram studies revealed between 2 to 7 bands, each with different characteristics. Higher frequency of the slow-moving esterases associated with high resistance was recorded in Mampong DBM population than the Accra suburbs. This finding confirmed results for carboxylesterase activity. Deltamethrin and cypermethrin demonstrated high negative correlation between fold resistance and carboxylesterase activity for β-isozyme. Chlorpyrifos-methyl, on the other hand showed positive correlation between fold resistance and α-carboxylesterase activity. The high carboxylesterase activities recorded for Mampong DBM, however, did not correlate with results of susceptibility studies. It is suggested that farmers should adopt an integrated approach using the IPM principles coupled with judicious use of biopesticides such as Bacillus thuringiensis.