Temephos [0,0'-(thiodi-4,1-phenylene) bis (0,0-dimethyl phosphorothioate): A13-27165] applied by ground equipment at 9.2 g/ha provided up to 98% control of mosquito species in El Salvador, Central America, in 1976. Aerial applications of the same compound were made at a rate of 8.3 g/ha in 1976 and 1977 to Anopheline breeding areas in a 150-km2 area along the Pacific Coast. In 1978, laboratory bioassays confirmed that An. albimanus had acquired a 164-fold cross resistance to temephos. This resistance might have resulted from yearly exposures of these mosquitoes to chemicals for cotton insect control in the area. Such resistance might be lost within a few generations if the population is no longer under pressure from temephos or related compounds.