Seven fractions of alpha(l-->3)-alpha-D-glucan from Ganoderma lucidum have been studied by light scattering, sedimentation equilibrium, and viscometry in dimethylsulfoxide (DMSO) containing 0.25 M lithium chloride at 25 degrees C. The intrinsic viscosity [eta] - molecular weight relation for this glucan in the mixed solvent is found to be represented approximately by [eta] = 0.071 M-w(0.60) cm(3) g(-1) in the range of weight-average molecular weight M-w studied, i.e., from 8 x 10(3) to 4.4 x 10(5). Its analysis based on current theories for wormlike chains shows that, without excluded volume effect, the (1-->3)-alpha-D-glucan chain is characterized by a linear mass density of 380 nm(-1), a Kuhn segment length of about 3 nm, and a diameter of 1.2 nm and is somewhat more extended but more flexible than amylose, a (1-->4)-alpha-glucan, in DMSO.