A series of copolymer latices synthesized from pyrrole and N-methylpyrrole were prepared with similar particle sizes. After a detailed characterization of the particles the systems were prepared as electrorheological (ER) fluids in dodecane with the particles stabilized by a graft copolymer with poly(12-hydroxystearic acid) as the stabilizing moieties. The shear stress-shear rate behavior of the systems was studied over a shear rate range from 10(-3) to 10(3) s(-1). An ac electric field at 50 Hz was applied to the fluids using field strengths up to 1 kV mm(-1) and the viscometric behavior examined. The particles with the highest conductivity exhibited Bingham behavior, and the fluid behavior could be calculated from the interparticle polarization forces. The Mason number provided good scaling of the data. The yield behavior was well described by the model due to Anderson and the high shear behavior by the Dougherty-Krieger equation. The fluids prepared with particles of lower conductivity showed pseudoplastic behavior with a power law index that was a function of conductivity, and the Mason number did not provide such a complete description of the viscometric response. The fluid with a particle conductivity of similar to 10(-9) S cm(-1) showed no ER response.