The report of physiologic resistance to DDT in Anopheles sacharovi Favre in 1951 was the first indication of resistance in a malaria vector. Today more than 35 species are resistant to DDT or dieldrin. Fourteen species are resistant to both compounds. A loss of susceptibility to both insecticides is found in several major vectors, such as A. sacharovi in Europe, A. albimanus Wiedemann in Central America, A. stephensi Liston in the Middle East, A. sundaicus (Rodenwaldt) in Indonesia and A. gambiae Giles in Africa. Resistance to DDT and dieldrin relates to 2 independent genetic factors. In some instances the resistance factor is dominant; in others, the hybrid individuals may also be as susceptible as the susceptible parent. The latter is more characteristic of DDT resistance, which fact partially explains its slow development as contrasted to the explosive nature of dieldrin resistance. On the whole, physiologic resistance has not been a major handicap to the overall malaria eradication efforts except in relatively restricted situations such as A. stephensi in the Middle East. Resistance to DDT and dieldrin can be overcome by resorting to organophosphorus compounds, but the latter course not only augments the cost of malaria eradication but also involves a higher level of technical competency and administration than necessary heretofore.