Fragmentation of forests has led to the creation of forest patches that differ in size, proportion of edge habitat, and degree of isolation. Although densities of many mammalian species are positively related to patch area, there appears to be a general negative relationship between density of white-footed mice (Peromyscus leucopus) and patch area. In Chapters 2 - 5, I investigated both resource-based (i.e., vegetation characteristics) and dispersal-based (i.e., inhibited dispersal and sink) hypotheses to explain the negative density - area relationship using live-trapping data and DNA-microsatellite analyses. As an extension of how forest fragmentation may affect movements by this habitat generalist into and out of habitats, in Chapter 6 I focused on how patch isolation influences population genetic structure. Data were collected from 1999 to 2001 in 15 different patches of various sizes and degrees of isolation. I confirmed that relative abundance of P. leucopus was negatively related to forest patch area (Chapter 2). My results indicated that relative abundance of P. leucopus was positively related to structural complexity of understory vegetation and total basal area of trees, and tended to be negatively related to the species richness of trees (Chapter 2 and 3). Although I was unable to differentiate whether understory vegetation results in higher relative abundances due to food or cover, the results suggest that vegetation characteristics contribute to the negative density - area relationship. Based on live-trapping data and DNA-microsatellite analyses, I rejected both dispersal-based hypotheses to explain the effect of patch area on relative abundance of P. leucopus (Chapters 4 and 5). Within-population genetic variation was high across populations, and only 50% of individuals could be assigned to their population of capture based on multilocus genotypes, suggesting high rates of gene flow. Degree of isolation and landscape structure had a small but significant influence on population genetic structure in this species (Chapter 6). This project illustrates the habitat generalist behavior and good dispersal ability of white-footed mice, but it appears that both abiotic and biotic factors, including competition and predation, are important in explaining the effects of forest fragmentation on P. leucopus.