Excised cochleae of alligator lizards were prepared to permit microscopic observations of motion of hair-cell free-standing stereociliary bundles and of the underlying basilar papilla during acoustic stimulation, using stroboscopic illumination. In response to tones of frequency from 0.2 to 5 kHz, the papilla rocks about an axis parallel to its length, displacing stereociliary bundles in the morphologically predicted direction of hair cell sensitivity. The papilla moves in phase along its entire length; for frequencies above about 3 kHz, the amplitude of motion of the most basal region is several times larger than that of the rest of the papilla. Over the basal two-thirds of the organ, stereociliary bundles stand freely in endolymph. In this region, maximum bundle height gradually decreases from about 30 to 12 micron; phase vs. frequency characteristics of bundle displacement with respect to the underlying papilla are those of nearly critically damped mechanical resonators. Resonant frequencies measured along the papilla vary inversely with a power (between 3/2 and 2) of bundle height and are close in value to auditory nerve fiber CFs measured in vivo at corresponding locations across the nerve. We suggest that length-dependent mechanical tuning of stereociliary bundles determines neural frequency selectivity and tonotopic organization in this part of the organ.