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The susceptibility of Culex pipiens larvae to some insecticides commonly used in mosquito control programs ]cypermethrin (Sparkill ® ), deltamethrin (Embrator ® ), temephos (Temepest ® ) and spinosad (Tracer ® )[ was investigated, also variations in esterases and glutathione S-transferases (GST) activities among three field populations (Abou homoss, Nadi El Said, Montaza) were measured and compared with a laboratory susceptible strain (S). The highest level of resistance against the tested insecticides was found in Abouhomoss strain (27.3, 22.2 and 24.8-fold) and the lowest level of resistance was recorded in Montaza strain (12.75, 15.17 and 8.17-fold) towards cypermethrin, deltamethrin and temephos, respectively. On the other hand, all strains recorded no resistance against spinosad (Tracer®). All field strains revealed significantly higher levels of GST and esterases activities compared with the laboratory susceptible strain. The results of the present study suggest that esterases and glutathione S-transferases enzymes have major role in Culex pipiens resistance to the evaluated insecticides.