Artificial selection on strains of Aedes aegypti showing susceptibility and refractoriness to oral infection with yellow fever virus (YFV) suggests that there is a significant genetic component to this trait. Using a population with an average susceptibility of 15%, inbreeding of isofemale lines followed by individual selection produced susceptible (29% infected) and refractory (11% infected) lines. The difference between lines was largely apparent before individual selection, which failed to increase/decrease susceptibility significantly. The findings suggest that very few loci with a major bearing on the trait segregated genetic variation in the original population sample, and that non-genetic factors also play a major role in determining whether or not Ae. aegypti females become infected with YFV.