Frequency- threshold tuning curves were recorded in thousands of auditory-nerve fibers in chinchilla. Synthetic tuning curves with 21 characteristic frequencies (187 Hz-19.04 kHz, spaced every 1/3-octave) were constructed by averaging individual tuning curves within 2/3-octave frequency bands. Tuning curves undergo a gradual transition in symmetry at characteristic frequencies of 1 kHz and an abrupt change in shape at characteristic frequencies of 3-4 kHz. For characteristic frequencies 3 kHz, the lower limbs of tuning curves have similar slopes, about -18 dB/octave, but the upper limbs have slopes that become increasingly steep with increasing frequency and characteristic frequency. For characteristic frequencies > 4 kHz, tuning curves normalized to the characteristic frequency are nearly identical and consist of 3 segments. A tip segment, within 30-40 dB of characteristic-frequency threshold, has lower- and upper-limb slopes of -60 and +120 dB/octave, respectively, and is flanked by a low-frequency ("tail") segment, with shallow slope, and a terminal high-frequency segment with very steep slope (several hundreds of dB/octave). The tuning curves of fibers innervating basal cochlear sites closely resemble basilar-membrane tuning curves computed with low iso-velocity criteria. At the apex of the chinchilla cochlea, frequency tuning is substantially sharper for auditory-nerve fibers than for available recordings of organ of Corti vibrations.