Certain forms of Anopheles gambiae s.s. actively maintain malaria transmission in the driest areas and months of the year because of considerable drought tolerance. We monitored desiccation resistance of F₁ offspring of both the M and S forms of field-collected An. gambiae s.s. Our results indicate that the geographic cline in the distribution of the two forms, as observed in Mali, corresponds to a physiological difference in response to arid environments. In addition, female mosquitoes survived significantly longer than males, enhancing the vector competence for the malaria parasite. Our study supports a genetic link to the drought tolerance phenotype, a phenotype with important consequences to malaria transmission in many places in Africa.