Six strains ofMusca domesticaL. from different geographical areas were intercrossed in combinations of 2 to ascertain whether any cases of cytoplasmic incompatibility or hybrid sterility could be isolated. No classical cases of either type were found; however, a significant depression in fertility was observed in several crosses, and 2 of the 6 strains showed gross behavioral or genetic differences. Some males from a strain of African origin would not or could not copulate with females of other strains, and males from an Australian strain produced a preponderance of male progeny in crosses with other strains.