Dental manifestations of the mild, transient osteopetrosis in the rat mutation microphthalmia blanc (mib) were examined. Eruption of all teeth was delayed in mib rats compared to normal littermates. The delays ranged from 5 days for incisors to 3 and 2 days for the first and second molars. Normal rats had straight incisors in the sagittal plane that exhibited signs of wear, but in mib littermates the incisors were maloccluded, distorted, and showed no signs of wear. Radiographic and histological examination of the dentition of 1- and 4-week-old rats revealed that the apical end of incisors in mib rats failed to extend posteriorly to the third molar region as in normal siblings, but ended at the first molar. Histological examination of longitudinal sections of mandibles through the incisors of neonatal normal and mib rats showed that in 1-day-old mutants the incisor was closely surrounded by alveolar bone to which it was ankylosed. The incisor body in mib rats was also malformed, with an indented apical end. This ankylosis was temporary, being resolved by 3 days. These findings show that neonatal reductions in bone resorption cause incisor defects and delay the eruption of all teeth in mib rats. The malocclusion and distortion of incisors of mib rats are likely caused by temporary ankylosis of incisor matrices to alveolar bone. Taken together, these findings illustrate the concept that bone resorption is an essential and rate-limiting element of tooth eruption.