In the remodeling interdental septum of the mouse, four types of Sharpey's fibers were observed. Classification of these fiber types was based on characteristics of their termination within the septum in relation to the resorption-related reversal line separating old and new bone. "Severed fibers" were located only within old bone and terminated at the reversal line. "Arborized fibers" were located only within new bone and terminated therein. "Adhesive fibers" were located within new bone and terminated within a heavy band of granular material at the reversal line. "Continuous fibers" had components within old and new bone. These components were connected across the reversal line by nonstriated fibrils. Adhesive fibers were the least numerous type; severed and arborized fibers were observed at nearly equal frequency. Continuous fibers were the most numerous type, their numbers being significantly greater than any of the other types (P less than .001). Mean numbers of continuous fibers were greater than the mean total of the three other fiber types (P less than .001). Mean numbers of severed, adhesive, and arborized fibers were not statistically different. The study suggested that continuous fibers could be transalveolar--that is, ones which pass through the septum without interruption joining fibers of the adjacent periodontal ligament. Maintenance of their spatial continuity appeared to require a connecting protein to orient new unit collagen fibrils to old ones in areas of reversal. Thus, transalveolar fiber bundles could be characterized as being composed of old and new segments joined by a connecting protein. As their unit collagen fibrils did not cross resorption-related reversal lines these fibers were spatially continuous but temporally discontinuous.