Following detection of West Nile virus in Harris County, TX, pyrethroid-based adulticides are applied to quickly to disrupt mosquito-borne disease transmission. However, populations of its primary vector, Culex quinquefasciatus, are resistant to permethrin-based adulticides in reference to the susceptible Sebring strain (SEB). Previous studies had detected high kdr allele frequencies in Cx. quinquefasciatus from the operational control areas in Harris County, TX. This mutation provides the advantage of overcoming pyrethroid exposure but may result in a fitness cost under low insecticide selection pressure. We considered that the pyrethroid resistance level may decrease during the months of December through April when adulticide applications by Harris County Mosquito and Vector Control Division are rare. We therefore collected Cx. quinquefasciatus from four areas to estimate their resistance ratio to permethrin-based adulticides using vial assays before and after three generations reared under laboratory conditions in the absence of insecticide selection. Our results demonstrated that mosquitoes from these four areas had low to moderate resistance to the permethrin-based adulticide. The PBO synergist ratio indicated that CYP450s contributed to permethrin resistance. RR₅₀s of mosquitoes from A109 before and after three months in an insecticide-free environment were not significantly different, suggesting a low probability of reducing permethrin resistance after three months without exposure to insecticides and partially explaining the current persistence of resistance populations in the field.