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Understanding microhabitat use is needed to make sound conservation decisions for at-risk, patchy-habitat specialists. Rock habitats offer unique microclimatic refugia for wildlife. Eastern Small-footed Bats (Myotis leibii (Audubon and Bachman, 1842)) use rock roosts during the summer, but data on these summer roosts are lacking for this species classified as (critically) imperiled in several US states and Canadian provinces and as globally endangered. Our goal was to characterize the structure and microclimate of M. leibii roosts at the southwestern periphery of their range. We predicted that M. leibii roost temperatures would be warmer and less variable than ambient temperatures and that solitary bats would use horizontal roosts cooler at night, whereas maternity group roosts would be vertical and warmer at night. During summers 2019 and 2020, we recorded physical (e.g., width) and temperature attributes of 58 M. leibii roosts at 16 sites in the Ouachita Mountains. Crevice roosts of M. leibii had narrow dimensions like elsewhere in their range and roost temperatures (measured with iButtons) were warmer and more variable than ambient temperatures. Group roosts were larger and had more stable temperatures than solitary roosts. These findings may be useful for assessing population threats, monitoring roost suitability, identifying roost-rich areas that need protection, and even planning artificial roost structures where natural roosts are limited.