A procedure to locate autosomal male-determining factors (AM) of the housefly is described, which employs a female-determining factor (F) marked with a closely linked dominant gene. A total of 70 AM chromosomes extracted from four IIM strains, nine IIIM strains and three VM strains of different geographic origins were examined.On the basis of female recombination frequencies, two to three types of AM chromosomes were found in each of the linkage groups, suggesting that the AM chromosomes are polymorphic for chromosome aberrations.All the AM factors were mapped to a specific site or closely linked sites on the respective autosomes. From the comparison of the mitotic autosome figures and the female recombination maps of AM chromosomes, it is suggested that AM factors are located near the centromeres of autosomes. The validity of this technique and the possible mechanisms for the origin of M polymorphism are discussed.