Physical differences were noted in the inner and outer parts of the tibial cortex of a sheep drinking water containing 100 parts per million of fluoride. In this animal's cortex and in the cortex of sheep fed one and ten parts per million, the fluoride content of the mineral was higher than that of a control sheep and higher in the outer than in the inner part of the cortex of the animals receiving fluoride. X-ray diffraction of the mineral phase after ashing demonstrated apatite in all animals and in those receiving ten and 100 parts per million, a second mineral phase, whitlockite, located in the outer cortical bone.