Island size, habitat heterogeneity, and distance from major ("mainland") stands of habitat were examined relative to composition and number of coexisting reptile species dependent on upland habitats of 11 mountain and 4 riparian habitat islands. Species richness increased with area on mountain islands, but area was unimportant in predicting species richness on riparian islands. Instead, isolation was of primary importance. Regardless of factors determining species richness, composition of species were deterministic; small assemblages were always totally included subsets of all larger assemblages. This pattern of determinism apparently reflects selective extinctions and the inability of species to recolonize due to the insurmountable barrier imposed by the Sonoran Desert.