Bat mortality caused by terrestrial wind-power plants has been documented and offshore wind-power developments may have similar effects. Determining which bat species occur offshore, how far they range from shore, and predictors of high activity may be helpful to developers and wildlife managers. We studied bat activity off the mid-Atlantic coast, using ultrasonic detectors mounted on ships in spring and fall 2009 and 2010. We investigated the association between nightly bat activity and weather variables, including wind speed, air temperature, and barometric pressure. Echolocation passes of bats totaled 166; maximum detection distance from shore was 21.9 km, and mean distance was 8.4 km. Most passes were identifıed as Lasiurnis borealis (Eastern Red Bats), representing 78% of bats identifıed to species or species group. Bat activity decreased as wind speed increased, but activity did not differ with distance from shore. Offshore wind projects proposed for locations beyond the maximum detection distances noted in our study would likely have few impacts on seasonal movements; however, depending on their location and operating protocols, projects closer to shore could result in fatalities similar to those reported at onshore wind facilities.