The ear of the mustached bat (Pteronotus parnellii) shows marked cochlear resonance near 60 kHz and many sharply tuned neurons throughout the brain have best frequencies (BF) near the cochlear resonance frequency (CRF). Controlled changes in the normal physiological range of body temperature (approx 37-42 degrees C) were used to change the CRF and to study the tuning properties of neurons in the cochlear nucleus (CN) and inferior colliculus (IC). In all cases there were concomitant shifts in the CRF and the BFs. Results were the same for single and multi-units, and for CN and IC units. Although the BF reliably changed with shifts in the CRF, the majority of the units showed no change in minimum threshold or the sharpness (Q10 dB) of tuning. The temperature-induced effects on cochlear tuning were similar to those previously described in nonmammalian vertebrates. The physiological data reveal that, within a narrow frequency band, cochlear and CNS tonotopy are labile in the mustached bat. The lability of tuning is further substantiated by adaptations of biosonar emission behavior with shifts in CRF (Henson et al., 1990).