Samples of compact bone from 9 animals were examined, all but one fossils. As an example of modern, recent bone, the Indian elephant (Elephas indicus) was used. There were 2 specimens of the mammoth Elephas primigenius, 2 of Mammuthus columbi (these 4 in varying states of preservation , and 1 of Gomphotherium of early Pliocene. In addition there was a sample of Uintatherium (Eocene), of an unidentified species of the Therapsida (Triassic), and of Brontosaurus (Jurassic). All specimens were cut in thin sections (17-300 microns) and examined under the microscope with both ordinary and polarized light. The 6 elephant and mammoth bones were also subjected to contact microradiography. The bone histology of the normal elephant is perfectly preserved in mammoth removed from ice in Alaska, and mammoth from the La Brea tar pits, although each of these bones is slightly discolored with brown pigment from the soil. Incipient accumulation of birefringent mineral can just be detected in a mammoth bone from an open site in Alaska; heavy mineralization is seen in a mammoth bone of much earlier provenience from Irvington, California, and in a bone from Gomphotherium. Maximum, and complete mineralization is found in the Therapsid bone from the Triassic and the Brontosaurus bone from the Jurassic. Mineralization of two types is found, one consisting of the incorporation in toto of discrete external fragments, the other the deposition of crystals from soil solution in the bone. The fine organization of secondary bone tissue is preserved throughout extended geological time. The individual Haversian systems may be distinguished as such in the oldest bones examined. The pattern of concentric striations imposed upon the living osteon by the fibrillar organization of the collagen is still visible under polarized light in the bones from the Triassic. This implies a substitution of mineral matter for the original organic matter at the submicroscopic, or macromolecular level. In normal mammalian bone under polarized light a sharp differentiation between the outer and inner areas of an osteon on the one hand and the intermediate area on the other hand, is clearly evident. This feature is found in the Indian elephant, the frozen Alaska specimen, and the mammoth from Irvington, California. It is drastically reduced, or is entirely absent from all other bones studied.