Since the appearance of DDT-resistance in houseflies in 1947, over 30 mosquito species have developed resistance to this and other commonly used insecticides. Much knowledge has been gained concerning the mechanism of resistance in insects but, in general, this phenomenon cannot be explained in terms of a single factor common to all resistant species.Enzymic detoxication of DDT to DDE has been found in the study reported to be a major factor in the resistance of several mosquito species, particularly Aedes aegypti. However, such correlation could not be established in a highly DDT-resistant strain of Culex pipiens fatigans originating in Delhi, India. Furthermore, resistance of the latter strain to o-chloro-DDT and o,p-DDT cannot be attributed to a detoxication process since these compounds are refractory to dehydrochlorination both in vivo and in vitro. Quantitative data on metabolism of DDT derivatives are given and other possible mechanisms of resistance are discussed.