Twelve active roosts containing 27 Myotis leibii were located in and near the Badlands National Monument. South Dakota. Active and inactive roosts were characterized as to temperature inside and outside of the roosts, position and size of opening, slope facing, internal dimensions, and soil type. Two roosts each contained more than two individuals; all others held single individuals or a single female and her young. A possible instance of twinning was observed. Young bats were characterized as to weight, length of forearm, amount of pelage, and flying ability.