The ring-opening polymerization of 3-O-benzyl-beta-L-arabinofuranose 1,2,5-orthopivalate (1) by BF3.Et2O gave stereoregular 3-O-benzyl-2-O-pivaloyl-(1-->5)-alpha-L-arabinofuranan with [alpha](D)(25) -151.4 degrees and a number-average degree of polymerization ((DPn) over bar) of approximately 91. These results indicate that the substituted effects derived from the chemical synthesis of cellulose by ring-opening polymerization can be also applied to that of the arabinose ortho eater derivative. Removal of the pivaloyl and benzyl groups gave a linear stereoregular polysaccharide (1-5)-alpha-L-arabinofuranan.