The anopheline mosquito is the target in most malaria control programs, primarily through the use of residual insecticides. A mosquito was studied that is refractory to most species of malaria through a genetically controlled mechanism. A strain of Anopheles gambiae, which was selected for complete refractoriness to the simian malaria parasite Plasmodium cynomolgi, also has varying degrees of refractoriness to most other malaria species examined, including the human parasites P. falciparum, P. ovale, and P. vivax for which this mosquito is the principal African vector. Furthermore, the refractoriness extends to other subhuman primate malarias, to rodent malaria, and to avian malaria. Refractoriness is manifested by encapsulation of the malaria ookinete after it completes its passage through the mosquito midgut, approximately 16 to 24 hours after ingestion of an infective blood meal. Fully encapsulated ookinetes show no abnormalities in parasite organelles, suggesting that refractoriness is due to an enhanced ability of the host to recognize the living parasite rather than to a passive encapsulation of a dead or dying parasite. Production of fully refractory and fully susceptible mosquito strains was achieved through a short series of selective breeding steps. This result indicates a relatively simple genetic basis for refractoriness. In addition to the value these strains may serve in general studies of insect immune mechanisms, this finding encourages consideration of genetic manipulation of natural vector populations as a malaria control strategy.