White-nose syndrome (WNS) has caused recent catastrophic declines among multiple species of bats in eastern North America1,2. The disease’s name derives from a visually apparent white growth of the newly discovered fungusGeomyces destructanson the skin (including the muzzle) of hibernating bats1,3. Colonization of skin by this fungus is associated with characteristic cutaneous lesions that are the only consistent pathological finding related to WNS4. However, the role ofG. destructansin WNS remains controversial because evidence to implicate the fungus as the primary cause of this disease is lacking. The debate is fuelled, in part, by the assumption that fungal infections in mammals are most commonly associated with immune system dysfunction5,6,7. Additionally, the recent discovery thatG. destructanscommonly colonizes the skin of bats of Europe, where no unusual bat mortality events have been reported8,9,10, has generated further speculation that the fungus is an opportunistic pathogen and that other unidentified factors are the primary cause of WNS11,12. Here we demonstrate that exposure of healthy little brown bats (Myotis lucifugus) to pure cultures ofG. destructanscauses WNS. LiveG. destructanswas subsequently cultured from diseased bats, successfully fulfilling established criteria for the determination ofG. destructansas a primary pathogen13. We also confirmed that WNS can be transmitted from infected bats to healthy bats through direct contact. Our results provide the first direct evidence thatG. destructansis the causal agent of WNS and that the recent emergence of WNS in North America may represent translocation of the fungus to a region with a naive population of animals8. Demonstration of causality is an instrumental step in elucidating the pathogenesis14and epidemiology15of WNS and in guiding management actions to preserve bat populations against the novel threat posed by this devastating infectious disease.