The mouse mutation fidget arose spontaneously in a heterogeneous albino stock. This mutant mouse is characterized by a side-to-side head-shaking and circling behaviour, due to reduced or absent semicircular canals. Fidget mice also have small eyes, associated with cell-cycle delay and insufficient growth of the retinal neural epithelium, and lower penetrance skeletal abnormalities, including pelvic girdle dysgenesis, skull bone fusions and polydactyly. By positional cloning, we found the gene mutated in fidget mice, fidgetin (Fign), which encodes a new member of the 'meiotic' or subfamily-7 (SF7; ref. 7) group of ATPases associated with diverse cellular activities (AAA proteins). We also discovered two closely related mammalian genes. AAA proteins are molecular chaperones that facilitate a variety of functions, including membrane fusion, proteolysis, peroxisome biogenesis, endosome sorting and meiotic spindle formation, but functions for the SF7 AAA proteins are largely unknown. Fidgetin is the first mutant AAA protein found in a mammalian developmental mutant, thus defining a new role for these proteins in embryonic development.