1. A high incidence of abnormalities of the maxillary incisor was found amongst aged mice in a colony of C57BL/Icrf mice. The anomalies were virtually absent in young adult and middle-aged mice of the colony.2.The frequency distrubution of affected animals in the samples of the aged population suggested a progressive increase in the frequency of defects after 24 months. 3. Hair impaction in the gingival tissues of both maxillary and mandibular incisors was found in mice of all ages. 4. Serial observations on a group of aged mice revealed that unilateral abnormalities of the incisor had no immediate deliterious effect on the mouse while bilateral anomalies tended to result in rapid loss of weight and death. 5. Investigations on the heads of a group of aged mice dying from natural causes demonstrated that the molar teeth are normal except for severe wear. 6. Radiological and histological studies of the same group of mice revealed many pathological features including the presence of secondary incisors, dens in dente and odontome-like structures. Loose hairs were commonly present in the gingival sulci and also deeply-embedded in the periodontal ligament of both normal and abnormal incisors