In 1973, 119 bats of three species were collected from four localities in Maryland and West Virginia. The collection included 43 big brown bats (Eptesicus fuscus), 43 little brown brown bats (Myotis lucifugus), and 33 eastern pipistrelles (Pipistrellus subflavus). The bats were collected from Round Top Mountain, Washington Co., Md.; Trout Cave, Pendleton Co., W. Va.; Montpelier Barn, Prince Georges Co., Md. Residues of sigmaDDT were highest in carcasses of bats from Round Top Mountain, which is surrounded by apple orchards. Bats from Trout Cave had the lowest residues, a circumstance which probably reflects the absence of agriculture and industry in the area. A polychlorinated biphenyl (PCB) and oxychlordane were highest at Montpelier Barn. Sources of the PCB are unknown, but chlordane is used against termites and in gardening at nearby housing developments. Residues in bats from North East Methodist Church were low except for dieldrin. Among species, little brown bats usually had the highest residue concentrations in their carcasses, whereas big brown bats had the lowest. When DDE in carcass fat of all species was above 60-90 ppm, it became measurable in brain tissue. Above 60-90 ppm, DDE levels in brains rose with increasing levels in carcass lipids. Residues of the PCB tended to respond similarly. Residue levels in brains were greatest in little brown bats; the maximum level of the PCB, 7.9 Ppm, was more than twice that of DDE.