Eight species of vespertilionid bats were identified during field surveys on the Sioux District, Custer National Forest in June and September 1994. Most of the 18 sites surveyed were associated with water (spring, reservoirs, beaver ponds, stock ponds). Species identified were long-eared myotis (Myotis evotis), western small-footed myotis (M. ciliolabrum), long-legged myotis (M. volans), fringed myotis (M. thysanodes), silver-haired bat (Lasionycteris noctivagans), big brown bat (Eptesicus fuscus), hoary bat (Lasiurus cinereus) and the Townsend big-eared bat (Plecotus townsendii). The literature on the bat fauna of the Sioux District includes one species not detected in 1994, little brown myotis (Myotis lucifugus) (Anderson and Jones 1971, Jones et al. 1973). Most Myotis species cannot be distinguished with bat detectors (the primary survey tool in 1994), so M. lucifugus may actually have been present (unidentified Myotis species were detected at nine sites). M. thysanodes (an adult non-lactating female), caught in a mist net in the Slim Buttes, is a new species recorded for the area. Current lists include 2 bat species from Chalk Buttes, 7 species from Ekalaka Hills, 8 species from the Long Pines, 6 species from North Cave Hills, 3 species from South Cave Hills, and 9 species from Slim Buttes; no survey work was conducted in Chalk Buttes in 1994. Only the list from Slim Buttes includes all species known to occur on Sioux District lands. Only Eptesicus fuscus has been recorded from each of the six main units of the Sioux District; with the exception of M. thysanodes, all other species have been detected on at least three of the six units. Seven species are known to breed on the Sioux District, based on males with enlarged testes, lactating females, or females with embryos. One other species (Plecotus townsendii) was present at sites in the Ekalaka Hills, Long Pines, North Cave Hills, and Slim Buttes during the breeding season, is known to breed in the Black Hills to the south, and probably breeds in the area. Only M. thysanodes, which also breeds in the Black Hills, is of uncertain status on the Sioux District at this time. Currently, nothing is known about which species overwinter on the Sioux District.