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An investigation was conduced to examine human physiological variables which influence the choice of host made by the malaria vector, Anopheles gambiae (species A from Nkolmekok, Cameroons). The experiments were conducted under conditions which simulated as closely as possible those under which female A. gambiae normally bite. Skin temperature, pigmentation, subcutaneous fat, age, sex, nutritional status and ABO blood group were determined in 102 human volunteers and analyzed as attractant factors. It was found that the ABO status of the blood meal taken could be reliably established by extracting the blood from the mosquito’s gut and performing direct agglutination tests on it with anti-A or anti-B sera. Analysis of the blood group data revealed that the mosquitoes preferentially selected hosts of blood group O. These findings are examined in relationship to the ABO balanced polymorphisms and known pressures operating upon this system. The results suggest that these disease vectors may act as agents of natural selection in the maintenance of ABO polymorphisms among human populations, particularly where the threat of malaria is a significant factor.