The skull of the Lower Triassic reptile Procolophon is described on the basis of newly prepared specimens in the collection of the British Museum (Natural History) and from casts made from high-fidelity natural molds. It is concluded that this genus had a large tympanum supported by the supratemporal, squamosal, and quadratojugal. The stapes remains incompletely known but probably functioned as part of an impedance-matching system comparable with that of modern lizards. The teeth of the upper and lower jaws are inset from the margin of the skull, suggesting the presence of cheeks. Jaw movement is confined to the vertical plane. Neither resorption pits nor weakly attached crowns provide evidence of tooth replacement. The position of the teeth is controlled so that the upper and lower teeth interdigitate exactly, with wear facets on the anterior and posterior surfaces. A natural cast of the nasal cavities resembles that of Triassic cynodont therapsids, suggesting the presence of turbinais and extensive olfactory epithelium. Procolophonoids may have evolved from the primitive, protorothyrid reptiles. They show no close affinities with other anapsids.