Winter activity patterns of bats in the western US are poorly understood. The recent introduction and subsequent westward spread of white-nose syndrome (WNS) has decimated hibernating bat populations in eastern North America and created an urgent need for scientists to understand basic information about bat ecology during the winter season. We used acoustic monitoring to document the activity patterns of bats outside of 3 hibernacula in western Montana from January through April, 2011. Acoustic monitoring has been suggested as a potential surveillance tool for detecting WNS; however, baseline information must first be collected to test this technique. We detected bat activity during every month of the study, with activity increasing each successive month. Bat activity was highly correlated with temperature at sunset; however, we detected activity at low temperatures similar to those reported in the literature (−3.4°C). Almost half (47.8%) of all bat passes occurred within 2 h after sunset, and we recorded very few bat passes during diurnal periods (0.1% of total). We also found variation in activity rates among the 3 sites. This study establishes that bat activity occurs outside hibernacula during the winter season in western Montana and at temperatures not typically associated with bat activity. We recommend additional acoustic monitoring to document the winter activity patterns of bats and further assess potential use of the technique as a WNS surveillance tool in western North America.