Ground sections were prepared of the seven root apices of five teeth preserved in the fossil hemi-mandible specimen, KNM-ER 1817, from Koobi Fora, Kenya. The sections were studied with transmitted polarised light microscopy. Despite the poor gross macroscopic preservation of the mandibular bone, the dentine microstructure was well preserved. Good details of typical dentine structure were observed in many of the sections including both short-period daily incremental lines and long-period lines. The spacing between consecutive daily incremental lines were compared with those published for dentine in modern humans and other primates. Close to the root surface, there appears to be a consistent rate of ~2.5 µm (micrometres) per day. Coarser accentuated lines (Owen’s lines) together with long-period lines were also clearly visible in the largest apical root section of the M3. These were used to reconstruct the slowing rates of root extension in the last 4 mm of the mesial M3 apex of KNM-ER 1817, which were then compared with those known for a sample of modern human molars.