Using 12‐ to 18‐month ‐old rats, we examined the ultrastructural and cytochemical features of multinucleated fibroblastic cells (MFCs) in the periodontal ligament (PDL) of molars. In aged rats, the MFCs were distributed randomly in the PDL and exhibted cytoplasmic structural variations which were not dependent on the number of nuclei. There was a tendency for the MFCs to cluster in the PDL. The MFCs, rich in cytoplasmic organelles involved with procollagen synthesis such a rought endoplasmic reticulum and the Golgi apparatus, incorporated and secreated3H‐prolince‐labled products. The MFcs also possessed many phagosomes containing intact collagen fibrils.n These MFCs were apparently involved in phagocytosis and intracellular degradation of incorporated collagen fibrils. Phagosome‐rich MFCs contain acid phosphatase activity in primary and secondary lysosomes, similar ro stronger in intensity to that which can be demonstrated in mononuclear fibroblasts. However, unlike mononuclear fibroblasts, the MFcs did not exhibit alkaline phosphatase acitivity along their plasma membranes. These results suggest that MFcs demonstrate a range of fibroblastic cellular activity, including collagen phagocytosis, and that they may lack certain plasma membrane glycoproteins, which might explain the occurrence of multinucleation in