ABSTRACT.Variation in susceptibility of the vectorAnopheles stephensiListon to the human malaria parasitePlasmodium falciparum(Welch) was demonstrated using twelve strains of mosquitoes and one strain of parasites culturedin vitro.The Beech strain ofAn. stephensiexhibited greatest natural refractoriness, but with high intrapopulation variability. By selection for the required characteristic, two refractory lines of the Punjab strain and one highly susceptible line of the Sind strain were obtained. The median number of oocysts in the two refractory lines was less than4%of that in the unselected line, whilst the highly susceptible line yielded about twice as many oocysts as the unselected line.Selection progressed more by keeping the descendants of individual females separate and selecting between them (individual selection) rather than pooling the progeny of all selected mosquitoes (mass selection). Using the former procedure many lines were lost due to inbreeding depression, but the outcome was more successful.