Bot flies are common parasites of Peromyscus leucopus, although determination of a cost to the host has been elusive. The goal of this study was to further explore the potential costs of bot fly parasites for a population of P. leucopus. We investigated the effects of parasitism on host condition (mass after controlling for parasite mass and host body length) and survivorship (the number of days animals persisted on trapping grids). Parasitism was quantified by prevalence (proportion of the population infected), intensity (the number of parasites per infected host), and dispersion of parasites within hosts (clumped, regular, or random). In addition, we searched for spatial and temporal patterns in infection. Finally, we analyzed the relationship between population demography and parasitism. Contrary to expectations, we found that infected mice persisted longer on trapping grids and were in better condition than uninfected mice. Also, we discovered that when considering overall infection levels, parasites were clumped within hosts, but when considering the number of simultaneous infections, parasites were randomly distributed among hosts. Although most animals had single infections, there was a high incidence of reinfections, leading to bimodal patterns of parasitism. Prevalence was not correlated with host density, sex ratio, or proportion reproductive, but there were significant relationships between intensity and density and sex ratio in 1 year. In addition, prevalence and proportion of reproductively active animals were asynchronous. These results suggest that bot flies do not impose an obvious cost to their hosts, and hosts may express some degree of tolerance for bot fly parasitism.