The gross morphology of the cochlear ducts of approximately half (150) of the living genera of lizards and a third (130) of the living genera of snakes have been studied. The differences in the structure of the cochlear duct are related to both the acoustical capacities and the taxonomic relationships of certain lizards and snakes. The cochlear duct of lizards consists of fairly well joined lagenar and limbic portions. By contrast, the cochlear duct of snakes consists of a lagenar sac somewhat constricted from the limbus. Each family of lizards has a morphologically characteristic cochlear duct, but taxonomic relationships are indicated by certain anatomical similarities. The cochlear duct of snakes is more primitive than that of lizards, and, unlike lizards, does not exhibit marked specializations of its various parts. Differences in morphology of the cochlear duct in snakes are much more related to habitat than family. The limbus and papilla basilaris of snakes regardless of family, are most elongated in bin rowing species, are only moderately elongated or ovoid in terrestrial species, and are small or reduced in certain arboreal and aquatic species.