When house fly,Musca domesticaL., populations were exposed to five insecticides (arsenite, dieldrin, malathion, methomyl, and fenvalerate) in different sequences and rotations, the development of resistance was slowed only slightly and cross-resistance was reduced compared with exposure to one insecticide. In only one population that evolved methomyl resistance was there evidence that it evolved significantly greater susceptibility to two insecticides (arsenite and fenvalerate). When a fly population was exposed to a combination of fenvalerate, a sticky trap, and a biocontrol agent (wasp parasite), it evolved resistance to fenvalerate, but not to the sticky trap or parasite. However, populations exposed to one control factor evolved resistance to the sticky trap, parasite, and fenvalerate.