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In Morocco, Culex pipiens plays a role in the high annoyance experienced by most urban cities, suburban and rural areas, especially since it was strongly suspected as the most likely vector in the transmission of West Nile virus epidemics that have hit Morocco in 1996. Chemical insecticides are generally the way in which they use the programs against harmful mosquitoes and disease vectors. However, the repeated and excessive use of these products regularly led to the emergence of the phenomenon of insect resistance. At the center of Morocco, information on the susceptibility or resistance to insecticides in mosquitoes (larvae and adults) vectors of diseases or pests, are almost nonexistent. This article reports the results of studies conducted between 2007 and 2010 with sensitivity tests WHO on larvae local populations of Culex pipiens collected in three lodging in the city of Fez, towards the insecticide mostly used by hygienic services: temephos. Five concentrations of insecticide (0.0025 mg/l, 0.005 mg/l, 0.0125 mg/l, 0.025 mg/l, 0.0625 mg/l) in addition to control, were used to determine the LC50 and LC 90 of Culex pipiens species towards temephos. Sensitivity tests were carried out at the entomology unit and monitoring of insect sensitivity towards insecticides installed at the Regional Diagnostic Laboratory Epidemiological and Environmental Hygiene (LRDEHM), Fez, under the Regional Directorate of Health in Fes Boulemane Region. The LC50 and LC90, concentrations corresponding to 50 and 90% mortality were determined graphically, by the linear relationship between the decimal logarithm of insecticide concentrations (x-axis) and the percentage of mortality transformed into probit values (ordinate) on logarithmic gausso paper. Resistance rates were determined on the basis of the sensitivity of a reference strain (S-Lab). The bioassay results affirmed the presence of resistance in larvae Culex pipiens towards temephos and that this species has also equally developed resistance levels similar and comparable in the three lodging studied, resistance rates recorded varying between 12.17 and 14.34. Facing such a situation, the surveillance of susceptibility of mosquitoes to insecticides used in mosquito control and anti-malarial fight has become imperative. This would undoubtedly allow a good management of the products available and consequently to adopt suitable measures for the best management of this resistance which must be an integral part of any program of vector control.