We sampled small mammals of fragmented sagebrush steppe on the Snake River Plain, Idaho, and compared the effects of habitat isolation on their diversity, abundance, and species composition in 2 landscapes, kipukas, which are patches of sagebrush-steppe habitat that were isolated by late-Pleistocene and early-Holocene lava flows, and remnant patches of sagebrush steppe that remain within recently developed agricultural areas. Species richness decreased with increasing isolation in both lava and agricultural landscapes, and Peromyscus maniculatus (deer mouse) was the only species found on kipukas that were isolated by more than 400–800 m, suggesting that many native sagebrush-steppe species may decline or disappear from fragmented sagebrush steppe. Density of small mammals increased with isolation on the agricultural patches, but decreased with isolation on kipukas; however, increased densities were entirely due to P. maniculatus. Diversity of small mammals on kipukas was highly correlated with absolute and proportional abundance of shrubs, probable indicators of sagebrush-steppe vegetation. Additionally, Bromus tectorum (cheatgrass) may play a role in the low diversity and density of mammals on kipukas, because both diversity and density were lower where cheatgrass was present than where it was absent.