Human and canine teeth have been subjected to X-ray examination. It has been found that the only crystalline constituent present in any appreciable amount, in enamel and dentine of both kinds of tooth, is apatite. The presence of additional material has been demonstrated by radiographs of tooth sections, and on the assumption that this matter is organic, its amount has been estimated from density determinations. It is thus estimated that enamel contains five per cent, by weight, of organic matter; and that dentine contains forty per cent, by weight. The densities of canine enamel and dentine were found to be slightly less than those of human enamel and dentine. The crystallites of dentine, of both kinds, have been found to be randomly oriented; but enamel, of both kinds, shows preferential orientation. The enamel crystallites, consisting of apatite, have the [001] direction in common, but are otherwise randomly oriented. The [001] direction makes an angle of 20° with the normal to the tooth surface in human teeth, and is coincident with the normal in canine teeth. The work was carried out on behalf of the X-Ray Committee of the Department of Scientific and Industrial Research, at the request of the Medical Research Council. The author is indebted to Dr. E. W. Fish, who has provided all the tooth specimens and sections. He is also grateful to Dr. G. W. C. Kaye, and Dr. G. Shearer, for their interest in the work.