The use of physico-chemical and microscopic technics shows that in carie-free fluorotic enamel well-ordered, closely packed, parallelepiped-like crystals can be found as well as hypomineralized areas in which the crystals are disordered and the interprismatic gaps widened. The observation of 1340 crystals shows an increase in their diameter measurements (600 to 700 A instead of 400 to 500 A) and alterations in their morphology, the faces of the crystals remaining nevertheless welldeveloped, in perfect agreement and their hexagonal symmetry. Together with the increase of the fluorosis index, it must be noted that fluoroapatite is progressively filling the place of hydroxyapatite. The coexistence of two different types of apatites within the crystals might be responsible for lattice dislocations involving a greater fragility of the central part of the crystals. The fact that fluoroapatite is very likely localized in the outer part of the crystals could be the reason why they are better resisting demineralization. The central holes within the crystals are not specific of fluorosis, they might rather by due to a demineralization carious process.