The effects of fragmentation may be variable for species that have seasonal patterns of density and reproduction. We tested whether the effects of fragment size and habitat on reproduction and density varied over the course of the breeding season for the white-footed mouse (Peromyscus leucopus). The relative density of mice changed; in small fragments densities were lower in spring, similar during the midsummer lull, and higher in autumn than in large fragments. Large fragments displayed the bimodal pattern of litter production previously reported for P. leucopus, but small fragments showed a relatively constant number of litters among spring, lull, and autumn reproductive periods. Although no differences were found between edge and interior habitats in density, litter production, and reproductive effort during spring and lull periods, all were higher in edge than interior habitats during autumn. Changes in effects of fragment size and habitat may be mediated by the influence of fragmentation on seasonally important abiotic and biotic factors.