In this article, the linear and nonlinear rheological properties of fumed silica suspensions are studied in the presence of a cationic polysaccharide, chitosan. Chitosan is shown to adsorb on silica aggregates, and the viscoelastic properties of chitosan/silica systems are shown to depend strongly on the chitosan concentration regime. For silica volume fractions greater than 2%, addition of chitosan in the semidilute regime leads to rheological properties attributed to the formation of a colloidal suspension of percolating silica aggregates that interact through hydrogen bonds mediated by silica and adsorbed chitosan; whereas addition of chitosan in the concentrated regime leads to rheological behaviors attributed to the formation of a chitosan entanglement network reinforced by silica aggregates that behave as fillers.