Differentiation of preodontoblasts to odontoblasts and preameloblasts to ameloblasts during development of the mouse mandibular incisor proceeds in a gradient from the area of the odontogenic organ, where undifferentiated ectomesenchymal and epithelial cells proliferate, toward the incisal tip where mature tooth tissues, dentin and enamel, are present. The freeze-fracture technique has been used in the work presented here to study cell membrane ultrastructure of preodontoblasts and preameloblasts at several stages of differentiation. At early stages of differentiation, cuboidal preameloblasts are joined together distally by numerous gap junctions. Relatively fewer junctions occur elsewhere on the lateral plasma membranes, but gap junctions frequently occur proximally between preameloblasts and stratum intermedium cells. As differentiation proceeds and the cells become columnar, distal and proximal junctions persist. Tight junctions, however, were not observed at any of the stages studied. Intramembrane particle concentration of the lateral preameloblast plasmalemma appears to increase as differentiation proceeds. Odontoblasts are also joined distally by numerous gap junctions which persist through later stages of differentiation. Although odontoblast cell processes were observed to project toward the preameloblast layer, no clear points of cell to cell contact or defined intercellular junctions between the two cell types were observed.