Carious lesions on the mesial and distal aspects of a RM1 of a young adult 'robust' australopithecine. Paranthropus robustus, are described. The specimen is from Member 1 sediments of the Swartkrans cave, which are dated to between about 1.8 and 1.5 million years before present. The mesial lesion, located cervically, had progressed further than the two distal lesions, which were situated along the cervix and in the enamel of the interproximal contact facet. This is the third specimen of P. robustus to be discovered with dental caries, but the other two cases were associated with occlusal and buccal enamel hypoplasia. Caries is present in less than about 3.0% of the approx. 116-128 P. robustus specimens at Swartkrans, and this incidence is noticeably lower than in the small Homo erectus sample from the same site. While this difference may simply be a sampling artefact, it may reflect dietary differences between these two synchronic and presumably sympatric early hominid species.