Dynamic (DLS) and static light scattering (SLS) were carried out on dilute aqueous solutions of silk fibroin samples collected from the posterior division of Bombyx Mori silkworms. DLS data identified the presence of the fast and the slow diffusive modes with diffusion coefficients different by 1 order of magnitude. The hydrodynamic radii RH were calculated from the fast and the slow modes by the use of the Stokes-Einstein relation as 16 and 240 nm, respectively. The z-average root-mean-square radius of gyration ([s(2)](z))(1/2) (=R-g) and the weight-average molecular weight, M-w, were determined from the SLS experiment as R-g = 330 nm and M-w 1.3 x 10(7), respectively. The fast and the slow diffusive modes were interpreted as being due to the translational diffusive motions of the single fibroin molecule and clusters of fibroin chains associated through ionic bonding between COO- ions of the fibroin molecules and Ca2+ ions remaining in the sample. The form factor of the cluster was calculated and compared with theoretical predictions for branched flexible polymers.