The wear resistance of single crystals of sapphire was determined as a function of crystallographic orientation with respect to plane of wear and rubbing direction. Distinct anisotropy was found. Evidence of plastic flow at the wear surface on both basal and prismatic slip systems was obtained. In bulk deformation tests, these systems do not become operative until a temperature of 1000°C is reached for basal slip, 1800°C for prismatic slip. The resistance against wear correlates in a qualitative way with the relative ease of operation of the two slip systems involved. When the orientation is such that both basal and prismatic slip are blocked, a brittle type of wear occurs that is characterized by conchoidal fracture. All results confirm that plastic flow during rubbing is a governing factor in, if not an integral part of, the wear process for even such a hard material as sapphire.