ABSTRACT:The toxic effect of commonly used insecticides in cotton fields was studied on 9 populations of Chrysoperla zastrowi sillemi (Esben-Petersen), an important predator of sucking pests collected in India. The dose mortality bioassay against 3-days old larvae was determined using three insecticides viz., endosulfan, fenvalerate and acephate by topical bioassay method. Mechanism of resistance to the above mentioned insecticides were determined without and with three metabolic inhibitors (synergists), viz., piperonyl butoxide (PBO), S,S,S-tributyl-phosphorotrithioate (DEF) and diethyl maleate (DEM). Among the populations, resistant ratios (RR) of CZS-8 was significantly higher i.e. 50.36., 66.11 and 277.51-fold for endosulfan, fenvalerate and acephate, respectively compared to susceptible population (CZS-10). The CZS-8 was selected for synergism study it showed higher LC 50 values and resistance ratio for all three insecticides. It showed 8.97-fold, 18.49-fold and 6.38-fold increase in synergism ratio for endosulfan indicating the resistance was strongly synergised by PBO, DEF and DEM. Similarly for fenvalerate, CZS-8 showed 8.69-fold and 3.63-fold significant increase in synergism ratio by DEF and DEM, respectively and for acephate, CZS-8 showed 54.82-fold, 150.87-fold and 113.52-fold significant increase in synergism ratio indicating that the resistance could be due to cytochrome p-450, esterase and glutathione s-transferase activity. The study indicated that the field population of C. z. sillemi developed resistance to different groups of insecticides. Among different geographical populations, CZS-8 collected from Sriganganagar, was recorded as most resistant.