AbstractFive cases of ameloblastic fibro‐odontoma were examined by light and electron microscopy. The ultrastructure of the epithelio‐mesenchymal interface was compared with the morphology of the normally developing tooth germ. It was noted that the ultrastructure of the tumors was identical with the normally developing odontogenic tissues up to but not including the differentiation of mesenchymal cells into tall columnar odontoblasts. The absence of odontoblasts in the ameloblastic fibro‐odontomas results in an absence of tubular dentin in place of which a homogeneous collagen‐containing material is synthesized. The failure of mesenchymal cells to acquire the morphology of tall columnar odontoblasts probably is due to the absence of the normally occurring induction of the mesenchyme by the odontogenic epi