This study was designed to observe the principal fibers and alveolar bone in various developmental stages in rat molars using light and transmission electron microscopy and to elucidate the mechanism of initial principal fiber attachment to the alveolar bone surface. Maxillary alveolar bone between the 2nd and 3rd molars of 20- and 25-day-old rats was used. A proteoglycan-rich, fiber-poor, and electron dense layer formed on the alveolar bone surface before the principal fiber organization. This layer was not seen before principal fibers had started to develop. Principal fibers first contacted and then became embedded in this layer. With further development, new bone deposited on this layer and around already attached principal fibers. These findings suggest that this electron dense, proteoglycan-rich layer may act as an adhesive factor to mediate the initial attachment of principal fibers to the alveolar bone surface.