Although bats have been observed from offshore ships, are known to cross large bodies of water, and have been observed seasonally on remote islands, little information has been collected regarding their distribution, species composition, and behavior at offshore locations. Techniques for monitoring bats over long periods are limited, and the typical logistical constraints for studies of nocturnal, flying species are amplified in open-water habitats. To our knowledge, this acoustic study represents the first attempt to monitor bats on a regional scale in an offshore environment. Long-term acoustic monitoring of 16 sites in the Gulf of Maine confirmed the presence of long-distance migratory and cave-hibernating bat species at coastal sites; large, forested islands; small, isolated rocks lacking woody vegetation; and an offshore weather buoy. Patterns of presence, species composition, and activity were highly variable among sites but consistently indicated that bats were most active and widespread between mid-August and mid-September, and that bats regularly occurred offshore. Sporadic presence of certain species during this period, surrounded by multiple nights with no activity, also suggests that bat presence offshore may be associated with migratory flight or use of remote islands as temporary roosts or stopover sites during seasonal movements.