As evidenced by domesticated species, generations in captivity can alter morphology in mammals and other taxa. This study examined morphological change in a captive wild mammal, the oldfield mouse (Peromyscus polionotus subgriseus). Specifically, cranial and mandibular size and shape were compared using geometric morphometric techniques. These data show that magnitude of change increased with generations in captivity, but physical changes between populations were not cumulative or progressive. Observed changes were likely due to relaxed selective pressures associated with captivity, coupled with founder effects. The results of this study have significant implications for captive management of mammals, particularly for those populations used for conservation. Biologists who bring animals into captivity need to be aware that morphological changes, as well as other changes, most likely have or will occur over generations in captivity. This, combined with changes taking place in wild populations, is likely to result in captive populations that are significantly different from their wild counterparts.