We used thermal imaging to estimate the sizes of the four major Brazilian free-tailed bat (Tadarida brasiliensis) maternity populations in Oklahoma during the summer of 2010 and 2011. The four Oklahoma maternity caves were Merrihew Bat Cave which averaged 66,708 bats, Reed Bat Cave which averaged 165,057 bats, Selman Bat Cave which averaged 54,238 bats, and Vickery Bat Cave which averaged 84,252 bats for all emergences. This study suggests there is a sizable difference in the thermal imaging population size estimates of free-tailed bats at Oklahoma maternity roosts compared to historical estimates using other methods. These differences might be due to the improved accuracy of thermal imaging, or because there is a real decline in bat numbers in Oklahoma, or both.