The ultrastructure of 3 fibrocartilages is described at the insertion of the adult rat Achilles tendon. Enthesial fibrocartilage lies at the tendon-bone junction, sesamoid fibrocartilage in the deep surface of the tendon where it presses on the calcaneus, and periosteal fibrocartilage covers the opposing surface of the bone. All had some features that could be interpreted as typical of fibrous tissues and other features that were more characteristic of cartilage. The general extracellular matrix was largely fibrous and had relatively little visible proteoglycan. In contrast, the cells had features more common in cartilage--dilated rough endoplasmic reticulum, glycogen and lipid, and pericellular matrix rich in proteoglycans and fine collagen fibrils. The periosteal fibrocartilage was the most 'cartilaginous' in character, probably because it develops rapidly after birth as a secondary cartilage from the calcaneal periosteum whereas enthesial and sesamoid fibrocartilages develop by metaplasia of tendon fibroblasts. A major difference between the 3 fibrocartilages was the arrangement of their collagen fibrils. There were parallel bundles in enthesial fibrocartilage but interweaving networks in the sesamoid and periosteal fibrocartilages. This probably reflects different functional demands of the tissues. The sesamoid and periosteal fibrocartilages directly formed the boundaries of the retrocalcaneal bursa and were not covered by synovium. Both were lined by an electron dense, articular surface lamina that prevented cell contact with the bursa. It significance is unclear, but it could protect against wear and tear, represent material being shed into the bursal cavity or control nutrient exchange with the synovial fluid.