STRO-1 is a cell-surface antigen. A cell population that is positive for the anti-STRO-1 antibody has been shown to contain mesenchymal stem cells. STRO-1-positive cells have been reported to reside in dental pulp and periodontal ligaments as well as in bone marrow. However, the tissue localization of STRO-1 in developing teeth is not clear. The present study was designed to investigate the spatiotemporal localization of STRO-1 in developing rat teeth by immunohistochemistry using light microscopy and electron microscopy. Wistar rats at ages 2, 3, 6 and 12 weeks postnatum and embryos at 18 days postcoitum were fixed with 4% paraformaldehyde. Mandibles and maxillae were resected and decalcified in 10% EDTA. The specimens were embedded in paraffin, and sections were cut and processed for immunohistochemistry using the anti-STRO-1 antibody. Selected specimens were frozen, sectioned and processed for immunoelectron microscopy. Immunoreactions for STRO-1 were identified in some bone marrow cells. Some odontoblasts and dental pulp cells showed positive immunoreactivity in developing rat tooth crowns and roots. Alveolar osteoblasts, cementoblasts and periodontal ligament cells were also immunoreactive. Electron microscopy localized the antigen in plasma membrane and some vesicles in dental pulp cells and odontoblasts. The study suggests that the STRO-1 antigen is involved in the differentiation of mesenchymal cell lineages and formation of the matrix in dental tissues.