Some mosquitoes became "resistant" to DDT. "There is persuasive evidence that antimalarial operations did not produce mosquito resistance to DDT. That crime, and in a very real sense it was a crime, can be laid to the intemperate and inappropriate use of DDT by farmers, espeially cotton growers. They used the insecticide at levels that would accelerate, if not actually induce, the selection of a resistant population of mosquitoes."
[Desowitz, RS. 1992. Malaria Capers, W.W. Norton & Company]
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"Resistance" may be a misleading term when discussing DDT and mosquitoes. While some mosquitoes develop biochemical/physiological mechanisms of resistance to the chemical, DDT also can provoke strong avoidance behavior in some mosquitoes so they spend less time in areas where DDT has been applied -- this still reduces mosquito-human contact. "This avoidance behavior, exhibited when malaria vectors avoid insecticides by not entering or by rapidly exiting sprayed houses, should raise serious questions about the overall value of current physiological and biochemical resistance tests. The continued efficacy of DDT in Africa, India, Brazil, and Mexico, where 69% of all reported cases of malaria occur and where vectors are physiologically resistant to DDT (excluding Brazil), serves as one indicator that repellency is very important in preventing indoor transmission of malaria."
[See, e.g., J Am Mosq Control Assoc 1998 Dec;14(4):410-20; and Am J Trop Med Hyg 1994;50(6 Suppl):21-34]
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