The authors have carrie out a study on the shape of apatite enamel crystals by means of field emission S.E.M.; for this purpose the lower incisor enamel of albino rats and the enameloid of fossil remains of Odontaspis from the Cretaceus and Miocene ages were used. The analysis of the available literature showed two different models: according to the first model enamel crystals were elongated hexagonal rods; according to the second, the one of Warshawsky, the enamel crystals were flat ribbons, with a rectangular cross-section. In accordance to the synthetic apatite morphology and to the shape of the unit cell, the authors show that the single enamel crystal is a rod with an equilateral hexagon base.