West Nile virus (WNV) infections in Turkey and East part of Europe have become one of the most important vector-borne diseases. Cx. pipiens L. complex species is a main vector species of the West Nile virus in these areas. Vector control practice with different classes of insecticides is the most commonly used alternative for mosquito control. The aim of this survey was to measure three different classes (Organochlorine, Organophosphate and Pyrethroids) of insecticide effectiveness, effects of synergist and possible biochemical mechanisms of Culex pipiens L. in Turkey. Bioassays results showed that permethrin and deltamethrin toxicity ranged from 50%(Mersin) to 84%(Huzurkent), whereas DDT toxicity was under 50% for all populations, except for Kapikaya (64%). In addition, Malathion toxicity ranged from 40%(Mersin) to 74%(Huzurkent). As a result of inhibitor assays, PBO and DEF increased the toxicity related to the insecticide classes. Biochemical assay results indicated that the MFO and NSEs (especially p-NPA) played an important role for DDT, malathion and pyrethroid resistance. GST activity assays implied that resistance was multifactorial for all tested populations.