Field populations of the tobacco cutworm, Spodoptera litura(Fabricius), showed resistance to commonly used insecticides. Development of resistance in the field populations to pyrethroids (cypermethrin, deltamethrin, ethofenprox, ethofenprox+PAP and fenvalerate) ranged from 100- to 2,700-fold, showing the highest resistance level to deltamethrin. Resistance to organophosphorus in secticides(chlorpyrifos, chlorpyrifos-methyl, EPN and pyraclofos) ranged from 2- to 32-fold with the highest level to chlorpyrifos. Resistance to carbamates (carbaryl and methomyl) ranged from 4- to 80-fold with the higher level to carbaryl. Detoxifying enzyme assays revealed that esterase and glutathione S-transferase activities were varied from 2- to 6-fold among the field populations. In addition, the bimolecular rate constants for inhibition of acetylcholinesterase by dichlorvos, eserine and monocrotophos showed 1.5-, 4.5- and 4.3-fold differences, respectively, between two different field populations. These results indicated that the broad spectrum of insecticide resistance observed in the filed populations was due to multiple resistance mechanisms, including increased detoxification of these insecticides and insensitive acetylcholinesterase.