During the last 12 years insert resistance to insecticides has become an increasingly serious problem throughout the world, and has been widely publicized from the standpoint of its effect on public health programs involving the control of insect vectors of human diseases. Very little publicity has been given to resistance in the common pests and vectors of diseases of livestock and poultry. Resistance in these pests is not yet such an acute problem in the (United States as in other sections of the world, but it is increasing each year. At present house flies (Masca domestica L.), numerous species of mosquitoes, the brown dog tick (Rhipicephalus sanguineus (Latr.)), the eat flea (Clenocephalides felis (Bouché)), and goat lice (Bovicola caprae (Curlt.) and Limbatus (Gerv.)) have been found resistant to one or more classes of insecticides. Reports have indicated that the chlorinated hydrocarbon insecticides are not as effective as formerly, and resistance is suspected but not definitely proved in the stable fly (Stomoxys calcitrans (L.)), horn fly (Siphona irritans (L.)), primary screw-worm fly (Callitroga hominivorax (Cqrl.)), cattle lice, and poultry mites. In foreign countries resistance has been confirmed in many species of mosquitoes, the stable fly, the sheep strike (Lucilia cuprina Weid.), and other blow flies, at least three species of fleas, and several species of ticks. Most of the resistant species can be controlled with organic-phosphorus insecticides, but several species of mosquitoes and the house fly have already become resistant to this class of insecticides.