The motor protein prestin in the outer hair cells is a prime candidate for the molecular amplifier that ensures the sensitivity, frequency tuning and dynamic range of the mammalian cochlea. Absence of prestin results in a 40-60 dB reduction in cochlear neural sensitivity. Here we show that sound-evoked basilar membrane (BM) vibrations in the basal cochleae of prestin-/- mice are as sensitive as those of their prestin+/+ siblings. BM vibrations in prestin-/- mice are, however, broadly tuned to a frequency ~ a half octave below the characteristic frequency (CF) of similar BM locations in prestin+/+ mice. The peak sensitivity of prestin+/+ BM tuning curves matches the neural thresholds, while prestin-/- BM tuning curves at the best frequency are > 50 dB more sensitive than the neural responses. We conclude that prestin influences properties of the cochlear partition that are crucial for BM frequency tuning and for converting its vibrations into neural excitation.