Resistance to insecticides among arthropods of public health importance has shown a steady increase since the initial report of resistant Musca domestica L. and Culex molestus Forskål in 1947. In the 1946–58 period, the total number of species reported as physiologically resistant to one or more pesticides was 46. Thirty of this number represented confirmed cases, including 20 species of mosquitoes (9 malaria vectors, 2 transmitters of encephalitis, one transmitter of yellow fever, and one vector of filariasis). In 1958, dieldrin resistance was reported for the initial time in Anopheles albimanus Wiedemann (El Salvador, Guatemala, Nicaragua), A. pseudopunctipennis Theobald (Mexico), and A. aquasalis Curry (Trinidad). In certain parts of El Salvador and Nicaragua, A. albimanus also is resistant to DDT. Except for resistance in Chrysomya putoria (Wied.) in the Belgian Congo, and in Phaenicia cuprina (Wied.) in Australia, blow fly species continue to remain susceptible to chlorinated hydrocarbon insecticides. Other species in which physiologic resistance has been confirmed are Pediculus humanus humanus L., Blattella germanica (L.), Cimex lectularius L., C. hemipterus (F.), Glyptotendipes paripes Edwards, and Culicoides furens (Poey). Behavioristic resistance to DDT is evident in A. albimanus (Panama), and in the United States, M. domestica has displayed similar resistance to malathion bait in a few localities. Resistance to both organophosphorus and chlorinated hydrocarbon insecticides has been demonstrated in M. domestica, C. putoria, G. paripes, Culex tarsalis Coq., and Aedes nigromaculis (Ludlow). Despite the increase in the magnitude of the resistance problem, many species continue to remain susceptible to DDT and other chlorinated hydrocarbon compounds even though under repeated exposure to these toxicants for periods of 5 to 10 years.