The results of these preliminary experiments with the house fly (Musca domestica) documented that sympatric populations under divergent selection with 5 to 30% migration between subpopulations can evolve and diverge. The fact that hybrids of the 2 strains showed no real preference for either medium indicates that a possible basis for selection for reproductive isolation existed in this experiment. Since these flies would deposit eggs with equal frequency on both banana and fish, they would be at a selective disadvantage compared with flies of either strain that exhibited preferences as high as 95% for the selected medium. A comparison of migration rates within and between the 2 systems indicated that each strain of fly has a different rate of movement. An important difference existed between the migration rates of the 2 systems when running under identical conditions. The 1st system had at this stage been subjected to selection for about 1 year longer than the 2nd system. There was some selection against emigration in this system as indicated by a relatively low migration rate. A female banana fly, for example, emigrating to the fish cell was at a decided disadvantage, because all her eggs placed in the banana vial of that fish cell would be destroyed.