This study presents data from 2 years of extensive sampling from July to October at 7 caves and mines used by bats in the Appalachian Mountains region of western Maryland and southwestern Pennsylvania. In total, 2,860 individuals from 7 species were captured. We examined species composition at roosts and compared our data to a survey conducted at 5 of the same caves and mines 2 decades ago; roosts were used by up to 6 species, and species composition has remained stable. In addition, we examined roost fidelity via mark and recapture (at 1 cave); population structure; and nightly, seasonal, and yearly patterns of use. Bats exhibited a very low rate of recapture (2.8%). Few bats were captured exiting the roosts before or during the 1st hour after sunset; peaks in nightly captures at roosts generally occurred 3–5 h after sunset, with few differences between sex and age classes. These patterns were indicative of frequent use of these caves and mines as night roosts by large numbers of individuals. For the 4 most abundant species (Myotis lucifugus, M. septentrionalis, Pipistrellus subflavus, and Eptesicus fuscus), roosts were used by adults and juveniles of both sexes. Adult sex ratios were skewed toward males, whereas juvenile sex ratios approached 1:1 in 1 of 2 years for each species. Nightly ambient temperatures explained little of the variation in capture times (2%) or capture success (10%). For each species, seasonal patterns of activity were similar between years, but there were differences among species. Overall, bat activity was highest in late August in both years.