This study documents developmental changes in the vocal reaction of bats to obstacles and describes the pulse patterns of young bats on their 1st and subsequent flights. The pulse repetition rate of young bats showed an increase just before the bats approached obstacles. This pattern differs from that of older bats. The distance at which the change in pulse repetition rate occurs is roughly twice as far from the obstacle in adults as it is in young bats. Even on their 1st flights young bats (25-29 days old) seemed to perform better than chance. This capability seems to be resistant to environmental manipulation during the preweaning period.