Although bats are well known for their use of ultrasound for echolocation, there is limited evidence for its use in a social context. We tested whether ultrasonic vocalizations in bats were contextually (roosting or flying) sexually dimorphic. During the reproductive season, we recorded ultrasonic signals of captive adult male and female big brown bats while the bats were flying on tether lines in the field, and compared these signals to ultrasonic vocalizations made while roosting in an anechoic chamber. Principal component analysis reduced 7 ultrasonic call descriptors to 2 components that related to frequency (PC1) and time or shape (PC2). While bats were roosting, ultrasonic call components related to time or shape and frequency were both sexually dimorphic, being increased in males in each instance. However, when bats were recorded while flying, these same call components were no longer sexually dimorphic. This finding suggests that bats are changing their ultrasonic calls in relation to functional context, making them monomorphic and utilitarian for activities such as foraging and navigation, but dimorphic in a situation when mating activity is likely.