In the house fly, Musca domestica L. (Diptera: Muscidae), sex is usually determined by a dominant factor, M, located on the Y chromosome. However, there are autosomal male (A(M)) populations in which the M factor is located on one or more of the five autosomes (I-V), most commonly on the third chromosome. Herein we report the use of isogenic strains to determine the relative fitness of Y(M) versus III(M) males in three different experiments. First, cages were started with 50% Y(M) and 50% III(M) males, and the frequencies of Y(M) and III(M) males were evaluated across generations. Second, mating competition studies were preformed with these isogenic strains. Third, the relative emergence rates of III(M) versus Y(M) male pupae held at three temperatures for 3 d were examined. All three studies indicate that III(M) males have a greater fitness than Y(M) males. In the cage competition studies, >90% of the males were III(M) after seven generations. III(M) males were more likely to mate than Y(M) males, and a higher percent of III(M) males emerged after being held as pupae at 4, 16, or 28 degrees C for 3 d. The implications of these studies to the distribution of III(M) and Y(M) males in field populations are discussed.