White-nose syndrome (WNS) is a cutaneous fungal disease of hibernating bats associated with a novel Geomyces sp. fungus. Currently, confirmation of WNS requires histopathologic examination. Invasion of living tissue distinguishes this fungal infection from those caused by conventional transmissible dermatophytes. Although fungal hyphae penetrate the connective tissue of glabrous skin and muzzle, there is typically no cellular inflammatory response in hibernating bats. Preferred tissue samples to diagnose this fungal infection are rostral muzzle with nose and wing membrane fixed in 10% neutral buffered formalin. To optimize detection, the muzzle is trimmed longitudinally, the wing membrane is rolled, and multiple cross-sections are embedded to increase the surface area examined. Periodic acid-Schiff stain is essential to discriminate the nonpigmented fungal hyphae and conidia. Fungal hyphae form cup-like epidermal erosions and ulcers in the wing membrane and pinna with involvement of underlying connective tissue. In addition, fungal hyphae are present in hair follicles and in sebaceous and apocrine glands of the muzzle with invasion of tissue surrounding adnexa. Fungal hyphae in tissues are branching and septate, but the diameter and shape of the hyphae may vary from parallel walls measuring 2 ??m in diameter to irregular walls measuring 3-5 ??m in diameter. When present on short aerial hyphae, curved conidia are approximately 2.5 ??m wide and 7.5 ??m in curved length. Conidia have a more deeply basophilic center, and one or both ends are usually blunt. Although WNS is a disease of hibernating bats, severe wing damage due to fungal hyphae may be seen in bats that have recently emerged from hibernation. These recently emerged bats also have a robust suppurative inflammatory response.