By submitting macroscopic (non-Brownian) monodisperse suspensions of solid spheres to an oscillating shear flow with a ‘‘rheo-optical’’ set-up, a quasi-periodic ordering of spheres has been observed during the shearing motion, and viscosity measurements have been performed simultaneously. The viscosity values corresponding to the ordered suspensions are significantly smaller than the values corresponding to the disordered suspensions; they are compared with viscosity calculations made by other authors in the case of periodic suspensions of particles.