Impulse activity of single auditory nerve fibres was recorded in the isolated half-head of the turtle in response to displacements of a piezoelectric probe placed on the basilar membrane. The temporal pattern of firing in response to sinusoidal displacements of amplitude 0.1-1.0 nm r.m.s. at a fibre's characteristic frequency could be matched to that generated by low-level tonal stimuli delivered to the tympanum. Frequency-threshold curves for acoustic and mechanical stimuli had similar shapes and differed only at frequencies above 500 Hz where the middle ear should filter acoustic but not direct mechanical stimuli. Step displacements of the basilar membrane gave a transient periodic discharge which resembled the responses to acoustic clicks. Most fibres initially increased their firing rate for rarefaction clicks and displacements towards the scala tympani.