Evaluation of fenitrothion resistance was realized in three populations of Culex pipiens collected in Southern Tunisia between March 2002 and October 2005. It was not possible to considered bioassays tests to fenitrothion in sample # 3 due to their control-level mortality. The RR50 were 27.1 in sample # 1 and 179 in sample # 2. All the studied samples showed the presence of one or more esterases in their electrophoretic profiles except the sample # 3 which was sensitive to propoxur. The addition of Pb to fenitrothion bioassays indicated the involvement of CYTP450 in the recorded resistance. This result could be explained by the massive use of the permethrin in the control against these insects in southern Tunisia. We also showed that the resistance to fenitrothion was correlated with the propoxur resistance. These results indicate that modifications of the target, AChE1, can be involved in the recorded resistance.