VESPERTILIONID bats produce FM echolocation pulses which sweep through a broad ultrasonic range1,2. Neurophysiological3,4 and behavioural5 evidence indicates that Myotis lucifugus, the most intensively studied of the vespertilionids, is maximally sensitive to the frequencies of its cries. The small size and mass of the middle ear structures in Microchiroptera6 suggest that the transduction system is specialized for the reception of ultrasound. It is obvious that the bat middle ear must respond to high ultrasonic frequencies, but it is not known how efficient the middle ear is, or whether this is achieved by tuned resonances or by a broad-band response. We have used the Mössbauer technique to investigate the contribution of the tympanic membrane to this high-frequency sensitivity. Although our primary interest was in the high-frequency response, we examined a broad range from 1 to 100 kHz.