Previous reports have shown expression of immunoreactivity for periostin, originally identified as osteoblast-specific factor-2, in the periosteum and periodontal ligament. However, the developmental changes in its expression and the detailed immunolocalization have remained veiled. The present study was undertaken to examine the spatiotemporal expression of this protein in teeth and their associated tissues of mice during development at light and electron microscopic levels. In tooth germs at cap stage, periostin immunoreactivity was recognizable in the interface between inner enamel epithelium and preodontoblasts as well as in the mesenchymal tissues around cervical loop. Dental follicles around tooth germs at bell stage localized periostin immunopositivity in addition to the immunopositive areas observed in cap-staged tooth germs, although the functional significance of periostin has remained unclear in tooth development. Furthermore, periostin immunoreactivity was also found in the alveolar bone surface. In the incisors of both 7- and 21-day-old mice, immunoreaction for periostin was discernible in the lingual periodontal ligament and labial fibrous tissue adjacent to the papillary layer. After postnatal day 7, immunoreaction for periostin came to be restricted to the fibrous bundles in the periodontal ligament in accordance with the organization of the periodontal fibers, indicating its localization matched the morphogenesis of the periodontal ligament. Immunoelectron microscopic observation of the mature periodontal ligament verified the localization of periostin between the cytoplasmic processes of periodontal fibroblasts and cementoblasts and the adjacent collagen fibrils. Our findings suggest that periostin is involved at the sites of the cell-to-matrix interaction, serving as adhesive equipment for bearing mechanical forces, including occlusal force and tooth eruption. © 2004 Wiley-Liss, Inc.