A model is developed for suspensions of rigid fibers in a non-Newtonian fluid exhibiting a yield stress by taking into account hydrodynamic and fiber fiber interactions. To be applied to fiber-reinforced viscoplastic fluids such as cementitious pastes or mortars, the matrix behavior is described by a Herschel-Bulkley law. The model is able to predict shear thinning and shear thickening behaviors as well as anisotropic yield stress depending on the fiber orientation. The extra stress term involves structural tensors, where exact analytical solutions have been proposed for an isotropic fiber orientation in simple shear flow. The model predictions show a good agreement with the yield stress values of steel fibers dispersed in kaolin pastes. (C) 2014 Elsevier B.V. All rights reserved.