Congenital thyroid disorders are often associated with profound deafness, indicating a requirement for thyroid hormone (T3) and its receptors in the development of hearing^1–5. Two T3 receptor genes, Trα and Trβ^6,7, are differentially expressed, although in overlapping patterns, during development^8–11. Thus, the extent to which they mediate unique or redundant functions is unclear. We demonstrate that Trβ-deficient (Thrb−/−) mice12 exhibit a permanent deficit in auditory function across a wide range of frequencies, although they show no other overt neurological defects. The auditory-evoked brainstem response (ABR) in Thrb^−/− mice, although greatly diminished, displayed normal waveforms, which suggested that the primary defect resides in the cochlea. Although hypothyroidism causes cochlear malformation^13,14, there was no evidence of this in Thrb^−/− mice. These findings suggest that Trβ controls the maturation of auditory function but not morphogenesis of the cochlea. Thrb^−/− mice provide a model for the human endocrine disorder of resistance to thyroid hormone (RTH), which is typically associated with dominant mutations in Trβ^12,15. However, deafness is generally absent in RTH, indicating that dominant and recessive mutations in Trβ have different consequences on the auditory system. Our results identify Trβ as an essential transcription factor for auditory development and indicate that distinct Tr genes serve certain unique functions.