A broad spectrum of resistance to organophosphorus insecticides has evolved in Culex pipiens quinquefasciatus (= fatigans) Say in California’s San Joaquin Valley. Resistance levels determined on F1 larvae following control failures by chlorpyrifos, involve Abate® (O, O′-(thiodi-p-phenylene) O, O, O′O′-tetramethyl phosphorothioate) at a level of 116.7 times the normal LC50, chlorpyrifos 52.2×, fenthion 48.9×, methyl parathion 24×, malathion 16.4×, and parathion 12.9×. Such multiresistance had appeareel earlier in 2 other major species, Aedes nigromaculis (Ludlow) and Culex tarsalis Coquillett, and is related to the multitude of chemicals employed in mosquito control and agriculture. Resistance in larvae is almost entirely overcome by the synergist DEF (S, S, S-tributyl phosphorotrithioate) but is unaffected by piperonyl butoxide. Thus, hydrolytic esterases and not mixed function oxidases are implicated as primary resistance mechanisms. Certain synthetic pyrethroids, derivatized carbamates, insect growth regulators, and a biodegradable DDT analogue have demonstrated high activity against the resistant population.