Bat houses are known to experience temperatures that can exceed suitable levels. For example, 40°C is considered an upper threshold for bat houses, and Tadarida brasiliensis can exhibit increased mortality above 45°C. Thus, artificial roosts could possibly become heat traps for bats and reduce survival. Yet, little is known about how bats adapt to environs that normally experience high temperatures, often exceeding 40°C. We monitored bat activity in northern California along the Sierra foothills. Twenty bat houses and the surrounding habitat were censused by guano counts and by acoustic recorders. We compared the daily temperature inside the bat house with ambient during 2023. For 51 days between April and September, the maximum roost temperature was above 40°C (42.55±0.28) and the ambient temperature averaged 38.31±0.42. The bat houses were on average 4.24° warmer than ambient. Bat activity decreased during the peak summer temperatures, indicating that the bat houses may be too warm for the bats. However, the decrease of bat house activity was also mirrored in some of the acoustic data, suggesting that this may be in response to the high ambient temperatures, and species may adapt by migrating into the foothills.