We surveyed the nonvolant mammals in 10 forest fragments embedded in a matrix of row crop agriculture in east-central Illinois to assess the impact of forest fragmentation on mammalian diversity and distributions. A total of 19 species were recorded during our study, including 16 native species that occur naturally in forest habitat. We found a significant species-area relationship and a significantly nested subset structure. In particular, gray squirrels, chipmunks and flying squirrels were only encountered in the larger, more continuous sites suggesting a negative effect of habitat fragmentation. Seven species were ubiquitous and we believe that several others occur periodically at all study sites, indicating that most mammalian species currently present have not had their distributions altered by changes in the intervening habitat. Though an analogy to oceanic islands may apply for some species, we believe that most mammals treat forest remnants as habitat patches rather than islands, and that mechanisms such as habitat selection, constraints due to home range size and differential dispersal ability best explain the observed distributions of mammals.