Genetic mosaics of two life-shortening (adult lethal) genes in Drosophila melanogaster were analyzed with respect to the location and extent of external tissues which exhibited the life-shortening genotype. It was hoped by this method to localize the site of action of each gene. For one mutant, AL 2, no correlation was found between the site of the adult lethal genotype and the adult life span of the mosaic fly. However, there was a direct correlation between the total amount of tissue exhibiting the adult lethal genotype and length of adult life. For the other mutant, AL 4, a more direct correlation could be made between location of adult lethal tissue and a shortened adult life span. Its focus was oriented toward the cephalic region. Like AL 2, it exhibited a direct correlation between total adult lethal tissue and length of adult life. The importance of this method of genetic analysis for the understanding of the action of life-shortening genes is discussed.