A genetic factor, Distorter, causes abnormal segregation ratios, resulting in the excess recovery of males in Aedes aegypti. The following factors were assayed to determine their influence on the production of high male ratios: differential mortality of immature stages, larval density, rearing temperature, parental age and genetic constitution. Genetic factors alone are responsible for high male ratios. The use of inbred lines has provided evidence to support the hypothesis that there are several forms of md with different levels of sensitivity to Distorter. These alleles produce different degrees of sex ratio distortion.