Genetic deafness is common, affecting about 1 in 2,000 births. Many of these show primary abnormalities of the sensory neuroepithelia of the inner ear, as do several hearing-impaired mouse mutants, suggesting that genes involved in sensory transduction could be affected. Here we report the identification of one such gene, the mouse shaker-1 (sh1) gene. Shaker-1 homozygotes show hyperactivity, head-tossing and circling due to vestibular dysfunction, together with typical neuroepithelial-type cochlear defects involving dysfunction and progressive degeneration of the organ of Corti. The sh1 gene encodes an unconventional myosin molecule of the type VII family. Three mutations are described, two mis-sense mutations and a splice acceptor site mutation, all in the region encoding the myosin head. The myosin type VII molecule encoded by sh1 is the first molecule to be identified that is known, by virtue of its mutations, to be involved in auditory transduction.