Population genetic theory indicates that a species' standing level of genetic diversity (GD) is explained in large part by the long-term effective population size (N_e). N_e depends not only on intrinsic species attributes but also on extrinsic factors such as environmental conditions that limit range sizes and promote or limit gene flow. We explored how various factors associated with N_e influence mammalian GD by first reviewing published estimates of mammal microsatellite variation and subsequently considering GD as a function of habitat type, trophic class, body size, range size, and conservation status. Our results indicate that microsatellite GD (i.e., heterozygosity and allelic diversity) is positively correlated with range size in threatened/endangered mammals although no significant effects of habitat, trophic class, or body size were observed. We also show that, on average, mammals have less GD than do birds or fishes.