Lattice imperfections in the structure of shark (Nataprion brevirostrais) enamel crystallites have been studied by means of lattice-imaging in a high-resolution transmission electron microscope. Line defects exhibiting various lengths (100-2500 A) and widths (10-30 A) and demonstrating phase behaviour different from that of the host crystal were observed along the fringes representing (100) lattice planes. When defect sites were subjected to an electron beam, an annealing process was observed in which the difference in phase contrast eventually disappeared and the defect itself became an intricate part of the (100)-type lattice planes. A through-focal series of the defects show a reversal in phase contrast. By examining the contrast behaviour under certain focal conditions and by adopting the Fresnel diffraction mode, the refractive index of the defect regions was determined to be lower than that of the apatite crystal. Present findings suggest that the interlayering structure along the (100) planes is responsible for the line defects observed in enamel apatites.