A survey of resistance in larvae of Culex quinquefasciatus conducted at three widely separated geographical areas of California, namely Coachella Valley, southeast Los Angeles and northern San Joaquin Valley, revealed high resistance to temephos and lower resistance to chlorpyrifos, methyl parathion and malathion at all locations. Likewise resistance to DDT was high in all areas despite its withdrawal from use since 1970. There was no obvious tolerance to propoxur. Tests for the presence of esterase-2A have shown a close correlation between the frequency of this esterase and the level of organophosphate resistance.