Throughout northwestern Ohio, northern Indiana, southern Michigan and western Ontario, only four records of the eastern pipistrelle (Pipistrellus subflavus) existed in the early 1980s. These individuals, three adult males and one bat of unknown sex or age, were considered to be vagrants or disoriented migrants. Between 1993 and 2006 we gathered 71 additional records of this species in the central Great Lakes basin, including reproductive females and young-of-the-year. Most were found near the eastern shore of Lake Michigan, particularly at two unusual hibernacula, Tippy Dam and Bear Cave. We speculated that modifications of Bear Cave associated with its commercialization in 1936–1940 created conditions suitable for the eastern pipistrelle to colonize this area.