We examined variability in bat sonar calls related to characteristics of the caller (i.e., individual identity, sex, state of lactation, age category, and colony membership) in 2 situations. If variation in call features reliably reflects characteristics of individuals, then there is potential for information transfer to conspecifics. Thus, sonar calls, in addition to their use in orientation and prey location, may function for communication. We obtained recordings of sonar calls from 66 little brown bats (Myotis lucifugus) captured from buildings at Chautauqua Institution, State University of New York at Fredonia campus, and surrounding areas in 2003–2005. Calls were recorded using a laptop computer, digitizing card, connector box, amplifier, and U30 bat detector while the bat crawled from hand to hand and while in flight. Calls were extracted and analyzed using custom computer programs to generate measurements for variables describing each call, which were then analyzed statistically. Our analysis revealed individual identity, state of lactation, and age category are reliably indicated by call variation of crawling bats, and individual identity information is contained in calls of bats in flight. To our knowledge these results are the 1st report of differences between sonar calls of lactating and nonlactating individuals for any bat species and individual differences in sonar calls for M. lucifugus. Individual identity information is available in calls produced within a situation; however, this information does not transfer well to the calls of the same bats produced in different situations. Our results suggest that the potential for communication of information via sonar calls depends on calling situation.