Hearing in mammals depends on a feedback process within the inner ear termed the 'cochlear amplifier'. The essential components of this amplifier are sensorimotor cells, the outer hair cells, which transduce motion of the basilar membrane induced by sound and generate forces to cancel the viscous damping of the cochlear partition. Outer hair cells alter the passive mechanics of the cochlea, enhancing both the sensitivity and the frequency selectivity of the auditory system. The molecular basis of the mechanism is thought to be a voltage-sensitive 'motor' protein, as yet unidentified, embedded in the basolateral membrane of the outer hair cell. The cochlear amplifier operates up to at least 22 kHz, but by measuring both the charge and mechanical movements associated with the motor in isolated membrane patches under voltage clamp, we show here that the limiting frequency at which the motor operates lies near 25 kHz. This value therefore sets an upper limit to the range of hearing in mammalian cochleas using this mechanism. The fast charge movement, arising from charge displacement within the presumed motor molecule, further suggests that the protein is more likely to be related to a transporter than to a modified ion channel.