The effects of dispersed drops in the electrorheological (ER) behavior of a polyaniline particle suspension were considered. Oil-in-oil emulsions, which differ in the electrical conductivity and dielectric constant, were employed for the liquid biphase. The yield stress behavior of ER suspension under steady shear and electric held was examined. Only when the dispersed drops with a higher conductivity formed the dispersed liquid phases in the presence of dispersed polyaniline particles was a synergistic effect in the yield stress observed, giving a better ER performance and reduced current density. A direct microscopic observation demonstrated that such a synergistic effect is due to the unique microstructures in a complex composed of particles and deformed emulsion drops. Not only the magnitude but also the dependence of the yield stresses on the electric field strength were strongly affected by the change in relative composition of particulate and liquid drop phases. The associated mechanism of ER response in particle-drop bidispersion was explained on the basis of the relationship between the microscopic change of structures and macroscopic rheological properties. (C) 2000 The Society of Rheology. [S0148-6055(00)00602-7].