Abundant fossilized remains of a medium-sized hyracoid from Arrisdrift, Namibia, are assigned to a new species of Prohyrax, P. hendeyi. The dental morphology indicates affinities with Parapliohyrax and Pliohyrax, and the genus Prohyrax is accordingly placed in the family Pliohyracidae (Pickford and Fischer, 1987). The absence of palatine pockets and external mandibular fossae indicate that the Arrisdrift hyracoid is a primitive member of this subfamily. The species P. hendeyi was about twice the size of the extant hyracoid Procavia capensis, on the basis of linear measurements of the skull and limb bones, and 30 per cent larger than Prohyrax tertiarius. Among the distinctive features of the postcranial skeleton of P. hendeyi in comparison with that of Procavia capensis are the less stable elbow (in flexed positions) and ankle joints, combined with more stable knee and wrist joints. These differences are considered to represent adaptations to increased terrestrial cursoriality in Prohyrax compared with Procavia. The occurrence of two mandibular size groups in the sample is thought to represent variability due to sexual dimorphism in Prohyrax. Even though the sample of Prohyrax tertiarius Stromer, 1922, does not permit extensive comparisons, it is suggested that P. hendeyi is closely related to it. Prohyrax differs from Meroehyrax Whitworth, 1954, of East Africa in its lack of internal mandibular chambers, absence of lingual fossae, and morphology of its cheek teeth. These differences refute Meyer’s (1978) suggestion that the two are synonymous.