Insecticide and resistance bioassays and microplate assays were performed on Culex pipiens mosquitoes to determine the level and mechanisms of resistance. Culex pipiens larvae were collected from three filariasis-endemic areas of Egypt and reared to adults for subsequent production and testing of F₁ generation larvae and adults. Bioassays were performed using World Health Organization (WHO) methods with the diagnostic doses of 6 organophosphate insecticides for larvae and 1 organochlorine (OC), 4 pyrethroid, 2 organophosphate, and 2 carbamate insecticides for adults. Microplate assays were performed to measure levels of beta esterase, acetylcholinesterase, insensitive acetylcholinesterase, oxidases, and glutathione-S-transferase enzymes. Larval bioassay results showed clear indications of resistance to organophosphate insecticides. Adult bioassays also showed widespread, significant resistance to many insecticides from all four classes, including the OC, DDT. The Qalubiya larval population was susceptible only to malathion, whereas Sharkiya larvae were susceptible to malathion, temephos, and chlorpyrifos. On the other hand, larval specimens from Assiut were resistant to all insecticides tested. Larval bioassay results were supported by those of microplate assays in showing elevated levels of glutathione S-transferase in populations from all three areas. In general, microplate results confirmed patterns of resistance observed using bioassays, and mechanisms of resistance were evident for all three areas sampled. Mechanisms of resistance are discussed in relation to microplate and bioassay results for the areas sampled and pesticides used.