At least some, and perhaps all, cases of calcaneonavicular bar are caused by a specific gene mutation which behaves as an autosomal dominant, probably with reduced penetrance. The available evidence suggests that this gene is distinct from each of the several genes responsible for other fusion defects of the bones of the foot and hand, such as talonavicular synostosis, talocalcaneal synostosis, the multiple fusions reported by Bersani and Samilson, and the two varieties of symphalangism. However, the available evidence concerning etiology of all of these conditions is scanty, and more data are needed. The examination of relatives of such patients, with roentgen examination of the feet and hands of apparently normal relatives, should prove to be rewarding and should furnish needed information concerning the cause and distribution of these defects.