Some preparations of xylan in the form of transparent membrance or amorphous powder prepared from deciduous woods such as beech and birch show a number of Debye-Scherrer rings at the irradiation with x-ray. Of special interest from the crystallographic standpoint is the diffraction pattern of the membrane, which shows an indication of a fiber diagram with the beam parallel to the surface, and this is more obvious when the membrane is stretched to an appropriate extent by rolling. The diagram can be neatly interpreted on the basis of an end surface-centered rhombic cell whose two axes of the base plane have exactly the ratio of , within the limit of experimental error. It is assumed that the cell has a trigonal or hexagonal symmetry. The observed reflections are indexed on the base of orthohexagonal unit cell, whose postulated three axes are a = 9.16 A., b = ., and c = 15.5 A. (fiber axis). The crystalline lattice of xylan is susceptible to mechanical treatment, heat, and moisture. The crystallinity depends greatly upon the uronic acid content. The diffraction pattern becomes less distinct and the interplanar spacings are enlarged with increasing uronic acid content, until at last the pattern diffuses into a single halo.