In this work, the structure of two kaolin sludges and a waste activated sludge are investigated using both light-scattering and free-settling methods. Fractal dimensions estimated by the light-scattering and free-settling techniques (D-S and D-F respectively) differ significantly and support the hypothesis that naturally occurring aggregates possess a multilevel structure. A two-level floc structural model comprised of (i) a primary floc (of fractal dimension D-S) consisting of primary particles and (ii) a secondary floc (of fractal dimension D-F) consisting of the microflocs is proposed to interpret the experimental findings. The structural changes of sludge floes before and after cationic flocculation are interpreted using the proposed two-level model.