Probable models for the crystalline structure of lentinan, the .beta.-(1 .fwdarw. 3)-D-glucan of the fungus Lentinus elodes, were evaluated by X-ray fiber diffraction and theoretical conformational analysis of single and multiple helices. The X-ray diagram is consistent with a hexagonal unit cell with dimensions a = b = 15.8 .ANG. and c (fiber repeat) = 6 .ANG.. Conformational analysis predicts 5 probable models for this fiber repeat, of which 1 is a single helix, 2 are double helices and 2 are triple helices. On the basis of packing of these helices into the unit cell and a comparison of their calculated Fourier transforms with the observed X-ray diffraction intensity distribution, all but the 2 triple helical models are eliminated from consideration. The 2 triple helices differ only in chirality and both left- and right-handed models are stabilized by interstrand O(2).sbd.O(2) H-bonds, such as are found in the triple helical structure of .beta.-(1 .fwdarw. 3)-D-xylan. The right-handed triple helix may be the more probable structure, by analogy with the known structure of the .beta.-(1 .fwdarw. 3)-D-xylan. It is likely that the .beta.-(1 .fwdarw. 3)-D-glucan of curdlan and those of Armillaria mellea and Lilium longiflorum possess the same structure.