Absolute hearing thresholds for two bats were determined by an operant conditioning technique. Pure tones rangingFrom 2.5 to 100 kilocycles per second were perceived by a single Eptesicus fuscus. Its maximum sensitivity was on the order of 68 decibles below 1 dyne per square centimeter and occurred at 20 kc/ sec. Maximum sensitivity for a single Myotis lucifugus was about 64 db below 1 dyne/ cm2 and occurred at 40 kc/ sec. The Myotis clearly heard a tone of120 kc/ sec but gave no evidence of hearing below 10 kc/ sec.