Insecticides are the most important component in the vector-control effort and pyrethroids such as permethrin are widely used for the indoor control of mosquitoes worldwide. However, the widespread development of resistance to pyrethroids is becoming a major problem. The current study reports an extensive survey of permethrin resistance in Alabama designed to characterize the importance of the L-to-F kdr mutation in pyrethroid resistant Culex mosquitoes. Mosquitoes were collected from 19 counties, 17 in Alabama and two from neighboring counties in Florida and Tennessee. Culex quinquefasciatus Say from all the counties tested were found to have developed resistance to permethrin. Seventy one percent of the field population from Alabama exhibited relatively low levels of resistance, with resistance ratios ranging from 71 to 390; 11% had medium/high levels of resistance, with resistance ratios ranging from 810 to 830; and the remaining 18% had high levels of resistance, with resistance ratios ranging from 1,100 to 1,400. Most of the mosquito populations tested showed a strong correlation between their level of resistance and the frequency of L-to-F mutation expression, suggesting the importance of target site insensitivity in the development of permethrin resistance. However, four populations with elevated levels of resistance showed no L-to-F mutation in their sodium channels. Although it is possible that other mutations other than L-to-F are present in the sodium channel, resulting in permethrin resistance in these four field populations, resistance mechanisms other than target site insensitivity also must be considered.