This study reports developmental changes in the vocal signals produced by wild-caught pre-volant and volant Myotis lucifugusAudio recordings were made from young animals (1-33 days old) and adults (over 1 year of age). The animals were removed from an attic maternity roost and studied individually in a room below. To stimulate flight-related behaviours, animals were released from a launching platform via a hinged floor, and their vocalizations were recorded as they approached a soft foam pad below. When the hinged floor opened, the youngest animals (1-4 days old) typically dropped onto the pad, but older animals either flapped their wings to achieve some horizontal displacement (>4 days) or sustained horizontal flight (>17 days). Vocalizations recorded under these conditions showed frequency modulation characteristic of adult echolocation sounds, even in animals as young as 4 days. Ontogenetic trends showed an increase in sound frequency, an increase in sound repetition rate and a decrease in sound duration as the animals matured. These data are discussed in the context of the development of echolocation behaviour in bats.