The teeth of sea urchins are connected to the calcareous jaw plates, known as pyramids, by a ligament consisting of collagen fibers and microfibrils synthesized by fibroblasts in the aboral growth zone of the tooth. This ligament needs to be sufficiently stiff to hold the teeth firmly when the animal scrapes hard surfaces, but also needs to be flexible enough to allow the teeth to move outwards during growth. To understand the mechanisms that regulate the growth and stiffness of sea urchin teeth, we have examined the ultrastructural organization of the supporting structures of Lytechinus variegatus teeth. Electron microscopy showed that collagen fibrils were mechanically attached to the column-like structures in the tooth, to produce a firm, irreversible connection. The ligament-attaching region in the jaws was formed by cavities that ramificated in the deep portions. The collagen fibrils were not mechanically linked to the jaws. These findings suggest that the stiffness of the ligament is mediated by chemical bonding between the collagen fibrils and the jaw surface. The cavities present in the pyramids greatly increased the surface area and strengthened the area for the bonding of collagen fibrils.