Abstract
Selection for early maturation applied to a laboratory colony of Musca domestica L. eliminated autosomally controlled DDT·resistance from both sexes, but a proportion of the males exhibited a genetically new type of resistance which was shown to be not transmitted through the females but to involve the y-chromo· some. By a single selection with DDT, applied to males only, the early-maturing strain was separated into two true-breeding strains omogeneous in both sexes with respect to DDT-tolerances, the one susceptible to DDT in both males and females, the other susceptible in females but showing at least an eightfold resistance to DDT in all its males.