A water-soluble glucan A was isolated from the fruit body of Auricularia auricula-judae. It is composed of a backbone chain of beta-(1-->3)-linked D-glucose residues, two out of three glucose residues being substituted at C-6 positions with a single glucose unit. The weight average molecular weight Mw, number average molecular weight Mn, and intrinsic viscosity [eta] of the fractionated samples were studied at 25 degrees C in water and in dimethylsulfoxide (DMSO). The Mark-Houwink equation was established as [eta] = 6.10 x 10(-4) Mw1.14 for the glucan A having Mw ranging from 9 x 10(5) to 1.6 x 10(6) in water. The values of [eta] in water are far higher than those in DMSO, but the values of Mn measured in water are the same as those in DMSO. Analysis of Mw and [eta] in terms of the known theories for rods and wormlike chains yielded 1030 +/- 100 nm-1, 90 +/- 20 nm, 1.3 +/- 0.3 nm, and 0.26 +/- 0.03 nm for molar mass per unit contour length ML, persistence length q, diameter d, and contour length h per main-chain glucose residue, respectively. The present data suggest that glucan A dissolves in water as single-stranded helical chains and in DMSO as semiflexible chains.