Particle-level simulations are employed to investigate the rheological properties of bidisperse magnetorheological fluids. These suspensions are treated as nonlinearly magnetizable, neutrally buoyant, non-Brownian spheres immersed in a nonmagnetizable Newtonian continuous phase. We examine the effects of particle size ratio, composition, and field strength on the dynamic yield stress. The dynamic yield stress of bidisperse suspensions is larger than that of monodisperse suspensions at the same particle volume fraction. The smaller particles cause the larger particle; to form more chainlike aggregates than those formed in monodisperse suspensions. (c) 2005 The Society of Rheology.