Kolombangara and Vella Lavella are about the same size and are separated by about 15 miles of water. _Rousettus amplexicaudatus_, _Pteropus rayneri_, _P. woodfordi_, _Macroglossus lagochilus_, and _Nyctimene albiventer_ have been collected on Kolombangara but only _P. admiralitatum_, _P. rayneri_, and _Dobsonia inermis_ have been found on Vella Lavella. The difference in the known megachiropteran faunas is more striking when one compares each island with adjacent islands. Two species on Vella Lavella occur also on Choiseul, which is about 35 miles northeastward, and two species occur also on Shortland, which is 120 miles northwestward (Fig. 16). Four of the five megachiropterans on Kolombangara also have been found on Choiseul, about 50 miles northward (Table 5). _Pteropus rayneri_ is the only megachiropteran known from both Kolombangara and Vella Lavella, even though the islands are separated by only a few miles of water. Inadequate data possibly account for the differences in the megachiropteran fauna, but I suspect that some other factors are involved. Although Vella Lavella and Kolombangara do have one species (_P. rayneri_) in common, a different subspecies occurs on each island--_rubianus_ on Kolombangara and _lavellanus_ on Vella Lavella (Fig. 17 and Table 6). This indicates that some factor or factors are operating to keep megachiropterans from moving frequently or easily from one island to the other. Each of several subspecies of species in the genus _Pteropus_ are known from one or two small islands separated by only a few miles from other islands on which different subspecies occur (see Fig. 6). Judging from this kind of distribution, these bats do not move frequently from island to island. Possibly this is because they cannot easily cross water barriers, or are not inclined to do so because food is abundantly available throughout the year on their home island. Because "flying foxes" frequently are seen in flight over water several hundred yards from shore, the first factor probably is unimportant--at least where short distances are involved. It seems most likely that when abundant food is available these bats have no reason to move even moderate distances. [Illustration: FIG. 17. The number of subspecies of megachiropterans known from individual islands (number within a circle) is compared with the number of subspecies common to different islands (number without a circle). For names of islands see Fig. 2.] Distributions of subspecies of polytypic species are summarized in Table 6 and Figure 17. Generally, more subspecies are known from the larger islands than from the smaller islands (Guadalcanal with 5, Bougainville, Choiseul, and Santa Ysabel with 4, Fauro with 2.) The distributions of some subspecies can be used to judge the differential effectiveness of water gaps between islands. The distribution of _Pteropus rayneri lavellanus_ and _P. rayneri rubianus_ is an example.