We evaluated hypotheses that the positions of day-roosting bats on the undersurface of bridges were governed by considerations related to predator avoidance and thermoregulation. From January 2002 to January 2003, we characterized bridge roosts in the Kisatchie National Forest located in north-central Louisiana, United States. Bats, predominately Corynorhinus rafinesquii, Pipistrellus subflavus, and Eptesicus fuscus, tended to roost in the darkest portions of the bridge and in the narrowest available spaces. They also roosted closer to the abutment, farther from the side edge, and closer to the ground than predicted; however, bats were never found roosting less than 0.4 m off the ground or against the abutments. Roost sites were warmer than other parts of the bridge and this difference was greatest in warm months. The small difference in temperatures between roost sites and other portions of the bridge is not consistent with predictions that day-roosting bats would choose the coolest possible locations. Tendency of actual roost sites to be slightly warmer than potential roost sites is best explained by the correlation of temperature with distance from the edge of the bridge. Our observations are most consistent with the hypothesis that bats roosted in areas that minimized their visibility and accessibility to predators.