Until recently, the little brown bat (Myotis lucifugus) was one of the most common bat species in North America. However, this species currently faces a significant threat from the emerging fungal disease white-nose syndrome (WNS). The aims of this study were to examine the population genetic structure of M. lucifugus hibernating colonies in Pennsylvania (PA) and West Virginia (WV), and to determine whether that population structure may have influenced the pattern of spread of WNS. Samples were obtained from 198 individuals from both uninfected and recently infected colonies located at the crest of the disease front. Both mitochondrial (636 bp of cytochrome oxidase I) and nuclear (8 microsatellites) loci were examined. Although no substructure was evident from nuclear DNA, female-mediated gene flow was restricted between hibernacula in western PA and the remaining colonies in eastern and central PA and WV. This mitochondrial genetic structure mirrors topographic variation across the region: 3 hibernating colonies located on the western Appalachian plateau were significantly differentiated from colonies located in the central mountainous and eastern lowland regions, suggesting reduced gene flow between these clusters of colonies. Consistent with the hypothesis that WNS is transmitted primarily through bat-to-bat contact, these same 3 hibernating colonies in westernmost PA remained WNS-free for 1-2 years after the disease had swept through the rest of the state, suggesting that female migration patterns may influence the spread of WNS across the landscape.