The unbranched homopolymer (1 --> 3)(1 --> 4)-beta-D-glucan shows a spontaneous gelation in aqueous solution above a critical concentration c(+). Detection of form birefringence, Deltan'(f), dichroism, Deltan", as well as their respective orientation, phi(f) and theta, in steady shear experiments with rheo-optical methods indicate the presence of aggregates in the sol state before the onset of the gelation. Transient dichroism experiments show that these aggregates are stable to shear forces. The aggregates show a finite deformability, their number is directly proportional to the polymer concentration, whereas the structure and deformability as well as the size of the aggregates is independent of concentration and molar mass above the critical concentration c Below c a change in the plateau level of the reduced dichroism Deltan"/c and therefore in the aggregate structure can be detected that coincides with the onset of a stable, nongelating state of the solution. From the intrinsic birefringence, which dominates at high shear rates, the stress optical coefficient C was determined to C = 1.19X10(-8) Pa-1. (C) 2003 The Society of Rheology.