The viscosity of silica suspensions in polyacrylamide (PAAm) solutions was measured as a function of particle diameter, molecular weight of PAAm, and mixing ratio of glycerin/water mixture as a solvent. The effects of polymer adsorption on the flocculation of particles and the flow behavior are discussed in terms of bridging. The suspensions of 10-nm silica are remarkably pseudoplastic. Such small particles are easily flocculated because only one polymer molecule can extend through many bridges. For suspensions of 20-nm silica, the irreversible increase in viscosity was often observed beyond the critical shear rate. This may arise from flocculation by shear-induced bridging in which the flocculate—flocculate bond is formed by adsorption of polymer extending from one particle to a particle in the other flocculate in shear flow. Since more than one polymer molecule is required to bridge large particles, the 40-nm particles are always in a dispersed state and their suspensions show flow behavior similar to that of the medium. With increasing molecular weight, the flocculating power of PAAm increases at first, shows a maximum at mol wt 2–5 × 10 6, and decreases. The flocculating power is enhanced in water because the adsorbed polymer is in an extended conformation in a good solvent.