The gap junction protein connexin30 (Cx30) is expressed in a variety of tissues that include epithelial and mesenchymal structures of the inner ear. We generated Cx30 (Gjb6) deficient mice by deletion of the Cx30 coding region. Homozygous mutants (Cx30^(−/−)) were born at the expected Mendelian frequency, developed normally and were fertile. However, they exhibit a severe constitutive hearing impairment. From the age of hearing onset, these mice lack the electrical potential difference between the endolymphatic and perilymphatic compartments of the cochlea, i.e. the endocochlear potential, which plays a key role in the high sensitivity of the mammalian auditory organ. In addition, after postnatal day 18, the cochlear sensory epithelium starts to degenerate by cell apoptosis. This degeneration process is likely to account for the concomitant decrease of the endolymphatic potassium concentration and the aggravation of the hearing loss in adult Cx30^(−/−) mice. The Cx30 ^(−/−) phenotype thus reveals the critical role of Cx30 both in generating the endocochlear potential and for survival of the auditory hair cells after the onset of hearing. The Cx30 deficient mice may represent a valuable model to study the mechanism of the hearing loss in human patients carrying a homozygous deletion of the CX30 gene (del Castillo et al., 2002, New Engl. J. Med., 346, 243–249).