Acoustic sampling for occurrence of the endangered Hawaiian hoary bat (Lasiurus cinereus semotus) was conducted at 12 locations on U. S. Army facilities on O‘ahu Island, Hawai‘i. Bats were confirmed as present at 10 of these locations: Dillingham Military Reservation, Helemano Military Reservation, Kahuku Training Area, Kawailoa Training Area, Mākua Military Reservation, Schofield Barracks East Range, Schofield Barracks West Range, Schofield Barracks (Mendonca Park Housing), Tripler Army Medical Center, and Wheeler Army Airfield. Our acoustic sampling did not record bat vocalizations at Fort DeRussy or Fort Shafter. Despite the presence of bats at the above 10 locations, foraging activity as identified from characteristic feeding buzzes was observed only at East Range and West Range of Schofield Barracks. Within-night bat activity pooled for all nights and detectors at each location showed bat activity was mostly confined to the first several hours of the night. Recordings were analyzed with Kaleidoscope version 4.1.0 (Wildlife Acoustics, Maynard, MA). All files were aurally and visually inspected for bat acoustic activity. The cumulative number of Hawaiian hoary bat echolocation pulses, call-events and feeding buzzes were summed for each monitoring station and night within a month.