The toxicity of permethrin to different life stages of Culex quinquefasciatus (Say) was examined in three field insecticide resistant strains, HAmCq^G0, MAmCq^G0, and BAmCq^G0; two permethrin selected strains, HAmCq^G8 and MAmCq^G6; and a laboratory susceptible strain, S-Lab. The levels of tolerance to permethrin differed among the developmental stages of Culex mosquito larvae with an ascending order from first to fourth instar. A developmentally regulated pattern of permethrin resistance in Culex mosquito larvae was identified, once again increasing from first to fourth instar. Adult mosquitoes had relatively lower levels of resistance in general compared with their larval stages, suggesting that different mechanisms may be involved in the response to insecticide selection pressure between larvae and adults. Inheritance of resistance to permethrin was examined using reciprocal crosses between resistant HAmCq^G8 and susceptible S-Lab strains; the resulting logarithm concentration-probit response curves for reciprocal F₁ lines were similar to those for the susceptible strain, with degree of dominance values of -0.2 to -0.3, suggesting that the inheritance of permethrin resistance in Cx. quinquefasciatus is incompletely recessive. No significant difference was found in the values of LC₅₀ and the slope of the log concentration—probit lines between reciprocal F₁ lines, indicating that the resistance to permethrin was inherited autosomally. Both χ² goodness-of-fit test and log concentration—probit line analysis on responses of backcross and self-bred progeny to permethrin indicated that permethrin resistance in the HAmCq^G8 strain did not follow a monogenic gene inheritance model, revealing that permethrin resistance in Cx. quinquefasciatus is controlled by more than one gene.