An investigation was undertaken to study the influence of a bimodal size distribution of particulates on the shear viscosity, primary normal stress coefficient, dynamic viscosity, and storage modulus of filled polymeric systems. As expected, all four rheological properties of the composite systems with a bimodal size distribution of solids were reduced in comparison to the properties of the composite systems with a unimodal size distribution of particulates. In addition, the maximum packing fraction was increased when the modality of the solids mixtures was increased. The present research effort showed that a design equation given by Ouchiyama and Tanaka and used by Gupta and Seshadri allows the a priori prediction of the maximum packing parameter as a function of particle size and size distribution. Consequently, it was possible to calculate an optimum reduction of rheological properties due to a bimodal size distribution of particulates that was in agreement with experimental results.