Insectivorous bats are an integral part of ecosystems because they consume significant quantities of nocturnal insects. White-nose syndrome is decimating populations of susceptible bat species in North America; thus, inventorying and monitoring bats are critical first steps in managing these important populations. We inventoried bat species at Boston Harbor Islands National Recreation Area (BOHA), MA, from 2010 to 2011. Using a combination of capture and acoustic methods, we documented 6 bat species, including Eptesicus fuscus (Big Brown Bat), Lasiurus borealis (Eastern Red Bat), Lasiurus cinereus (Hoary Bat), Lasionycteris noctivagans (Silver-haired Bat), Myotis lucifugus (Little Brown Bat), and Myotis septentrionalis (Northern Long-eared Bat). Bats occurred at all inventoried islands, although most activity of Northern Long-eared Bat, a species Federally listed as threatened, was documented at a mainland site in Worlds End, near Ice Pond. Although the full extent of bat use on the islands remains unclear, we provide evidence of bats roosting and foraging on the islands. During long-term acoustic monitoring at Thompson and Lovells Islands, we assessed the effects of weather and season on bat activity; the latter analysis provided evidence of bats migrating through the area in spring and autumn.