Habitat fragmentation is one of the greatest threats to the conservation of biodiversity and has 3 components: habitat loss, patch isolation, and patch size. We tested the effects of forest-clearing size on small mammal populations in the Upper Coastal Plain of South Carolina. These clearings act as islands for many species of small mammals, particularly old-field mice (Peromyscus polionotus) and cotton rats (Sigmodon hispidus). We live-trapped small mammals in 3 size classes of clearcuts (small <6 ha, medium 10-15 ha, and large >25 ha) on the Savannah River Site and compared relative density and diversity of small mammal species among the patch size classes. Peromyscus polionotus were captured on all grids and were the only species captured on small grids; P. polionotus relative density did not vary significantly among patch sizes. In contrast, S. hispidus relative density, and relative density of all species combined, tended to increase with increased patch size, although the differences in relative density among size classes were not statistically significant. Species diversity (H΄) increased with patch size and was significantly greater in large patches than in small patches. Thus, patch size may be an important factor affecting diversity of early successional species, but its influence varies among species.