Bats are being impacted by many threats (e.g., human disturbance of maternity roost sites and white-nose syndrome). Bat maternity roosts are a limited resource, and bats exhibit strong fidelity to these sites; therefore, identifying these features and conserving the surrounding habitat are important for bat management. Southern Idaho consists of the largest, contiguous volcanic pseudokarst area in the U.S., which provides important cave habitat for bats, but little is known about bat maternity roosts in this area. We compiled data spanning 36 years in southern Idaho to identify caves used by maternity colonies. From June 1980 to August 2016, researchers surveyed 313 caves. We documented 11 caves used by maternity colonies of Townsend's big-eared bats (Corynorhinus townsendii), and 10 caves that were potentially used by maternity colonies. In the 11 caves used as maternity roosts, the median number of Townsend's big-eared bats was 38, and the mean number was 49 individuals (SD = 47.1, range = 8 to 160 bats). Our data highlight southern Idaho as an important maternity roosting area for bats in Idaho and western North America, and provide baseline information of maternity caves prior to the arrival of white-nose syndrome, which can help managers during land-use planning in this area.