ABSTRACT The discovery of a new semi-dominant gene in the mouse is reported. It is called ‘dancer’, and it belongs to the linkage group XII. The homozygotes have cleft palate and cleft lip, and die soon after birth. The heterozygotes have ab-normal behaviour and a white spot on the head. An examination of the inner ear in the heterozygotes revealed a reduction of the vestibular ganglion, absence of some of its nerves, and widespread mal-formations in the vestibular region. Developmental studies showed that the malformations appear about 2 days after the abnormalities of the ganglion and nerves, and since they are virtually confined to the parts with affected nerve supply they are probably consequent on these abnormalities. A complete correlation between the defects of the semicircular ducts and of their nerves strongly suggests that the nerve plays an essential part in the morphogenesis of the proximal end of the duct. The failure of the otoliths to form in the utricle in the absence of the macula indicates that the otoliths are probably secreted by the macula.