Interparticle interaction and structure of glycogen from a shell fish (mussels) were studied in water and 0.5 N NaOH by static and dynamic light scattering and viscometry. A concentration regime up to 30% w/v was covered. Beyond the overlap concentration a large-scale clustering was observed which was interpreted as associates. Glycogen is a natural representative for hyperbranched polysaccharides. Interpretation of data was made for the inverse forward scattering (at scattering angle theta = 0). The measured quantities could be separated into interparticle interaction contributions and a change in the particle structure when the concentration was increased. A master curve was obtained for X < 1 when plotting M-w/M-app(c) against X = A(2)M(w)c = c/c*, where M-app(c) is the apparent (measured) molar mass at concentration c. This master curve agrees within experimental error with that for amylopectin. Deviations to a flatter curve were found when X > 1. Simultaneously to the forward scattering the apparent radius of gyration R-app(c) was measured. The influence of interparticle interaction could be split off, and the true R-g(c) was obtained. Up to X = 1 no change was observed, but at X > 1 a sharp increase in the dimensions by 2-3 orders in magnitude was found. Similar behavior was found also in dynamic light scattering where a slow mode of motion became noticeable at X > 2. These three effects-deviation from the interaction master curve, increase in R-g(c), and development of a slow mode of motion-give strong evidence for onsetting association. The specific viscosity showed common behavior with a weak increase in the dilute regime and a strong increase in the semidilute regime. The kink points were found with c[eta] = 0.57 and 0.82 for NaOH and H2O, respectively, much smaller than c[eta] congruent to 4 as observed for most linear macromolecules. However, a very good agreement with latex particles was found.