Computer simulations were carried out to investigate the shear resistance of electrorheological fluids, which are suspensions of electrically neutral particles in an insulating solvent, under ac and pulse electric fields. The shear stress of the system under a constant shear flow is found to pass through a minimum as the frequency of the electric field is increased. Two types of aggregation due to the induced dipoles are observed: at low frequencies the aggregation follows the applied field, while at high frequencies the particle clusters are unable to completely break up. This mechanism gives results which agree qualitatively with rheological experiments.