Toxic effect of sub-lethal concentration (LC20) of bifenthrin, chlorpyrifos and their combinations was studied on the specific activities of esterases in the 4th and 6th instar larvae of susceptible laboratory strain (Lab-S) and deltamethrin resistant population (GUW) of stored grain pest, Trogoderma granarium. After exposure to LC20 of bifenthrin, chlorpyrifos and their combinations of 3:1 and 1:3, the activities of total esterases, cholinesterase and carboxylesterase increased significantly in both the 4th and 6th instar larvae of Lab-S and GUW populations when compared with their respective controls. Acetylcholinesterase (AchE) activity was significantly decreased (28.94 and 42.42%) in the 4th instar larvae of Lab-S population after treatments with bifenthrin and bifenthrin: chlorpyrifos combination 3:1, respectively. After exposure to chlorpyrifos and bifenthrin: chlorpyrifos combination 1:3 the AchE activity was increased 45.24 and 64.28%, respectively. This increase was much more prominent in the 6th instar larvae viz., 93.95, 45.57, 182.52 and 132.61% following treatments with bifenthrin, chlorpyrifos, 3:1 and 1:3 combinations of two insecticides, respectively. In the resistant population this increase was 35.71, 48.10 and 75.61% in the 4th instar, respectively and 9.47, 28.17 and 60.29% in the 6th instar after chlorpyrifos, 3:1 combination and 1:3 combination treatments, respectively. This activity was decreased 63.99% in the 6th instar but increased in the 4th instar 13.36% after bifenthrin exposure. Arylesterase activity was declined after exposure to bifenthrin, chlorpyrifos and 3:1 combination in both the 4th instar(89.47, 64.33 and 36.65%, respectively) and 6th instar (84.49, 54.01, 4.28%, respectively) of Lab-S as well as in the 4th instar (77.73, 55.81 and 35.96%, respectively) of resistant population. This activity was upregulated (84.92, 141.86 and 192.88%, respectively) in the 6th instar of resistant population, following exposures to bifenthrin, chlorpyrifos and 3:1 combination. Whereas, this activity was increased in the 4th instar (53.22%) and 6th instar (124.60%) of Lab-S population as well as in the 4th instar (35.19%) and 6th instar (234.75%) of resistant population after treatment with 1:3 combination, respectively. The findings of current investigation suggests that insects upregulate the synthesis of esterases in general cholinesterase and carboxylesterase in response to exposure to insecticide to combat with the stressful conditions of exposure to toxic chemicals. The activity of the arylesterase, on the contrary is inhibited to variable extent after exposure to the insecticides.