The interfaces and the relationships between collagen and oxytalan fibers were observed under light and electron microscopy. Guinea pig periodontal ligament was prepared for light and electron microscopy with perfusion using Peter's buffered formalin for light microscopy and GTA-S-collidine and OSO4 for electron microscopic studies. The tissue for light microscopy was stained with a modified Gomori's aldehyde fuchsin technique, in which pre-oxidization with potassium monopersulfate was carried out before staining so as to demonstrate the oxytalan fibers. EM tissues were routinely stained with lead citrate and uranyl acetate. Two different structural relationships were observed. First, the subcomponents of the collagen and oxytalan fiber types interweave with each other; and, second, some of these two-fiber subcomponents appear attached to each other. These relationships and the known orientation of oxytalan fibers as seen in the periodontal ligament provide insight as to the function of oxytalan fibers. The oxytalan fibers may provide increased structural integrity and increased distribution of forces over a wider area of the periodontal ligament. Because of their close relationship to blood and lymph vessels in the periodontal ligament, they may also help to stabilize these elements by the same structural relationships to collagen fibers.