Viscosity and light-scattering measurements are performed on two dilute aqueous solutions of regenerated silk fibroin (RSF) of Bombyx mori silkworms obtained by dialysis against 6 M LiBr and pure water solutions at ambient temperature. The reduced viscosity of the RSF solutions in 6 M LiBr is found to decrease with decreasing polymer concentration C down to a certain concentration Cc but to increase below C, Static light scattering (SLS) for the same system shows that C/R-theta, where R-theta is the Rayleigh ratio, is at its minimum at near C-c. Dynamic light scattering (DLS) has revealed that silk fibroin molecules are present as a mixture of single molecules and aggregates in such a way that the majority of fibroin molecules are present as multichain aggregates above C-c and they start to dissociate below C-c, where most of them become isolated chains. Such a dissociation process of fibroin aggregates is not observed for the RSF solutions in pure water, in which multichain aggregates are dominantly present at higher C and persistently observed even at very low concentration. The aggregation behavior of RSF in both solutions is extensively discussed by combining SLS and DLS data.