A microbeam diffraction method in transmission electron microscopy has been applied to lattice structural studies of human and shark enamel apatite crystals. The technique allows diffraction patterns to be obtained from very small regions of the specimen (40 nm in diameter) and transfers minimal energy to the sample during analysis. The presence of crystal defects, nonisolated dislocations with the dislocation line positioned perpendicular to the 100-type planes, was observed in the central part of the crystals. Further experiments with acid-etching of crystals containing such defects showed an initial preferential dissolution at the site of these atomic imperfections. The relative abundance of the crystal defects was approximately the same in both human enamel and shark enameloid.