The spread of white-nose syndrome into western North America has caused concerns that bat species inhabiting the region will soon experience population declines like those seen to the east. However, much of the region lacks baseline estimates of abundance for bats believed to be susceptible to WNS due to a scarcity of known hibernacula, where populations are traditionally monitored. The goal of this study was to determine the abundance, roost fidelity, and activity patterns of summer colonies of female little brown myotis (Myotis lucifugus) in an area with no known hibernacula. To accomplish this, we subcutaneously implanted passive integrated transponder (PIT) tags into 297 female little brown myotis from 2015 to 2018 and installed radio-frequency identification (RFID) readers and antennas inside three maternity roosts in Yellowstone National Park. Bats occupied RFID-monitored roosts on 32% of days and were never detected at maternity roosts located > 16 km from where they were tagged. Roost fidelity varied throughout summer and was greatest during late gestation and early lactation. This allowed us to combine nightly scans of RFID-tagged bats with visual emergence counts to estimate the population of bats using two roosts located >16 km apart during 2017–2018. Population estimates of both colonies were markedly higher (N* = 208 ± 6 bats and N* = 836 ± 67 bats during 2018) than the number of bats seen during evening emergence (high counts of 127 and 222, respectively). Nocturnal visits of tagged bats to these roosts increased around the time of parturition, indicative of bats returning to nurse young. These data show that emergence count and PIT tag data can be combined to monitor bat population abundance and activity patterns in areas where hibernacula are absent or where their locations are unknown.