The sphenodontid genus Clevosaurus from the late Triassic of south west Britain is represented by at least two different species. Exceptionally abundant remains of Clevosaurus occur as part of a diverse assemblage of microvertebrates lodged within fissure deposits at localities in Glamorgan and Avon. Although the majority of specimens occur as beautifully preserved disassociated elements, certain associated and articulated remains have been recovered. Contemporaneous vertebrates include other sphenodontians, archosaurs and procolophonids. C. hudsoni is fully restored as a lizard-like reptile approximately 25 cm long. It was essentially insectivorous, although it may have also been facultatively herbivorous, breaking down food with a well-developed precision shear bite. A smaller species, C. minor, is less widespread and its remains are generally not as well preserved. It seems probable that a third species might be distinguished on the basis of dental characteristics, and the occurrence of a fourth species cannot be discounted. A distinct suite of apomorphic characters separates the Sphenodontida from other lepidosauromorphs and these apomorphies are readily identifiable in Clevosaurus. In certain late Triassic sphenodontians the lower temporal arcade was apparently incomplete, but, contrary to Robinson's (J. geol. Soc. Lond. 129, 457 (1973)) description, the lower temporal arcade was unbroken in Clevosaurus. However, the articulation between the jugal, squamosal and quadratojugal does not conform to the configuration exhibited by all other sphenodontians in which both temporal arcades are complete. In addition, the possession of a supratemporal in Clevosaurus is unique among known sphenodontians. The parietal table in Clevosaurus is intermediate between the broad, flat plesiomorphic condition, seen in forms such as Polysphenodon and Homoeosaurus, and the derived narrow median ridge displayed by Kallimodon and Sphenodon. Consequently, within the Sphenodontida the position of Clevosaurus is unclear; however, on the basis of the prominently flanged additional teeth, some reduction in tooth numbers and at least some partial reduction in the width of the parietal table, it would appear to share certain characteristics with Sapheosaurus and Kallimodon. Palaeopleurosaurus, described by Carroll (1985) as a primitive pleurosaur, may also have affinities with this grouping. Some of the earliest sphenodontians, such as Planocephalosaurus and Diphydontosaurus, exhibit fusion of the median skull roofing elements, a short temporal region and retention of a variable degree of pleurodonty in the marginal dentition. Together with Gephyrosaurus these genera may constitute a monophyletic assemblage forming the sister group of all other sphenodontians.