Fibers prepared from the enzymically debranched plant polysaccharide arabinan, containing (1-->5)-linked alpha-L-arabinofuranose residues, produce X-ray diffraction patterns containing concentric rings indicative of a high degree of structural organization. The measured unit cell dimensions and fiber density are consistent with a 2-fold helix passing through each unit cell. According to model-building techniques and energy calculations, a total of seven molecular structures (three with C2'-endo and four with C3'-endo sugars), all of pitch 8.74 A, and hence seven different crystal-packing arrangements are the most probable. While intrachain hydrogen bonds are only present in three cases, interhelix association is facilitated by hydrogen bonds in all allomorphs. Small energy differences between them suggest that easy conformational transitions may be made from one form to another. This polymorphism and the tendency to form microcrystals in the solid state might confer upon arabinan the observed fat mimetic properties.