An experimental investigation is undertaken into the shear-thinning behavior of suspensions of non-Brownian rigid fibers in Newtonian fluids. In particular, the influence of the shear stress and the fiber concentration is investigated. The shear stress is adjusted by varying both the shear rate and the solvent viscosity. In the semidilute concentration regime, where direct mechanical contacts between fibers are rare, the suspension is found to be nearly Newtonian over the stress range investigated. In the concentrated regime, the suspension becomes shear thinning below a certain shear rate. The shear thinning increases with concentration and decreases with solvent viscosity. Although shear-thinning behavior of fiber suspensions has often been reported in the literature, its physical origins are not well understood. Our experiments are interpreted in terms of the formation and breakage of fiber flocs due to the competition between hydrodynamic and colloidal forces. Our interpretation is confirmed by measurements of the adhesive forces between two individual fibers. (C) 2001 The Society of Rheology.