Biogeographic relationships of mammals occurring on mountaintops of the Great Basin of western North America are re-examined on the basis of newly acquired information. Contrary to previous indications, a relatively weak pattern is revealed between species richness and island size. The range of species numbers per mountain range varies only by a factor of two, with most islands having between 9 and 13 species, so that a species found on any one island is likely to be found on many, if not all, others. Analyses of species occurrences (nestedness, presence-absence patterns) also do not reveal strong effects of area. Consistent with prior studies, there is no discernible effect of isolation of mountaintops from one another or from putative mainlands; this is explicable from the disproportionate number of mobile woodland species and few sedentary subalpine forms among montane faunas. Although species and populations have disappeared from mountaintops since the late Pleistocene, they involve mostly cold-adapted subalpine species. Biogeographic processes shaping characteristics of modern mountaintop faunas in the Great Basin evidently involve a dynamic interplay between extensive intermountain dispersal and extinction resistance of remaining species. Brown's (1971, 1978) hypothesis, which posits that montane mammals in the Great Basin comprise remnant faunas formed exclusively by extinctions and no colonizations, is not supported.