The apparent hypermineralization which is often seen adjacent to canaliculi (∼ 2000 Å) and dentinal tubules in microradiographs may be the result of a reversal in orientation of microcanaliculi (∼ 500 Å) and adjacent mineralized fibrils at these sites. Microcanaliculi and adjacent mineralized fibrils, as seen by electron microscopic surface replication, extended at right angles from canaliculi for a distance of about 1–2 μ. In contrast, beyond this zone microcanaliculi were also observed but were oriented at right angles to the microcanalculi that were closer to the canaliculi. Although the significance of the difference in orientation of microcanaliculi in pericanalicular as compared with intercanalicular regions is not known, we demonstrated that when a porous material is x-rayed parallel to and then at right angles to the longitudinal axis of the porous phase an apparent difference in density is observed. These findings suggest that the apparent hypermineralization of bone adjacent to canaliculi described in former investigations utilizing microradiography may be the result of a difference in orientation of microcanaliculi and adjacent mineralized fibrils rather than a true variation in mineral density.