While bats have anecdotally been reported roosting in cliff swallow (Petrochelidon pyrrhonota) and barn swallow (Hirundo rustica) mud-nests for several decades, this roosting behavior is rarely discussed in the available peer-reviewed literature. The authors have documented at least seven bat species roosting in swallow mud-nests throughout California, Arizona, and New Mexico. Roosting bats have been observed inside swallow nests during the fall and winter seasons, and emerging from swallow nests during the spring and summer, indicating year-round use. The number of individual bats observed roosting in a single nest ranges from one to up to seven, and bats have been found roosting in up to 20-25 percent of nests at a single structure containing swallow mud-nests. At some sites, bat guano indicating use by bats has been observed in almost every swallow nest inspected. Bats can be injured or killed during the removal of swallow mud-nests from bridge and culvert structures during transportation construction projects. Given the extent of use by bats outside of the bird nesting season coupled with the widespread practice of removing swallow nests during that time of year, swallow mud-nest removal could have conservation implications for bats. Recommended best practices to minimize the potential for bat injury and mortality will be discussed.