Different types of chimerism result from double fertilization. Differences result from fertilization of first and second polar bodies and from early fusion of two zygotes. These three types of chimerism have different phenotypes or different proportions of possible phenotypes, and variation is greater in respect of gene loci close to the centromere. The phenotype frequencies in the different types of chimerism are tabulated and the phenotype frequencies expected in different mating types are shown. Studies of such cases offer the only method of autosomal gene localization relative to the centromere at present available in man.