The orientation relationship between apatite and organic matrix in shell of Lingula unguis (inarticulate brachiopod) was studied. The organic layers, mineralized layers, and decalcified mineralized layers were examined layer by layer using microbeam X-ray diffraction technique. Both organic layer and decalcified mineralized layer showed the diffraction pattern of beta-chitin. The degree of orientation of apatite showed correlation to that of beta-chitin: Well oriented diffraction patterns of apatite crystal and organic matrix were observed in the central part. In this part, the fiber axis of beta-chitin was parallel to the c-axis of apatite. A close relationship of unit cell dimension between apatite and chitin was indicated. These strongly suggest that the fibrous structure of organic matrix assists the orientation of apatite crystals in Lingula unguis shell.