A water-soluble glucan, namely, PD3, was isolated from the fruiting body of Dictyophora indusiata as the method reported previously. Its chemical structure was characterized by GC, FTIR, and (13)C NMR. The results indicated that PD3 is a (1-->3)-beta-D-glucan, with (1-->6)-beta-glucopyranoside side branches. The chain conformation and morphology of PD3 in aqueous solution were investigated by viscometry, rheometer, and laser light scattering (LLS) measurements, atomic force microscopy (AFM), and transmission electron microscopy (TEM), respectively. The weight-average molecular mass (M(w)), radius of gyration (R(g)), hydrodynamic radius (R(h)), and intrinsic viscosity ([eta]) of PD3 in water were determined to be 5.1 x 10(5), 141 nm, 44 nm, and 1440 cm(3) g(-1), respectively, by LLS and viscometry. The structural parameter rho (Rg/Rh) of PD3 was calculated to be 3.4, and the [eta] dependence of C(NaOH) of PD3 is similar to that of triple helical polysaccharides Scleroglucan and Lentinan, suggesting that PD3 exists as a triple helical chain in water. This conclusion was further proved by rheological measurement and AFM observation. Interestingly, the [eta] of PD3 dramatically decreased in a narrow range concentration of NaOH between 0.18 and 0.22 M, higher than that of Scleroglucan and Lentinan (both less than 0.1 M), indicating the helix-coil conformation transition of PD3 is more difficult than that of Scleroglucan and Lentinan. Moreover, with the increase of concentration, PD3 trends to self-assemble to fibrous aggregates in aqueous solution as measured by TEM.