In this paper, the colloidal stability and electrorheological responses of semi-conductive polyaniline particles dispersed in a mineral oil were investigated experimentally. The polyaniline was synthesized by polymerization of aniline monomer. The surfactants used for the colloidal stability were commercially available nonionic Span20, Span80, and Span85, which are distinctly different in their molecular structures. Dynamic yield stress of polyaniline suspension, which possessed the quadratic dependence on electric field strength at low electric field, exhibited the linear dependence at high electric field. In addition, the conductivity of polyaniline particles influenced the electrorheological response and there existed the optimal conductivity for the maximum yield stress. In the presence of nonionic surfactants used here, the yield stress of polyaniline exhibited a local maximum at a certain surfactant concentration. The existence of optimal surfactant concentration was confirmed by the surfactant adsorption isotherm. We hypothesized that below the optimal threshold concentration, the electrorheological activity was enhanced by the increased interfacial polarization induced by the adsorbed surfactant molecules. Above the threshold concentration, however, the electrorheological activity was degraded by the conduction through the field-induced surfactant-rich bridge between adjacent particles. This optimal concentration coincided with the saturation concentration in the adsorption isotherm. The surfactants used in this study showed the improved colloidal stability due mainly to the induced steric hindrance effect. For the colloidal stability Span80 was the most effective. However, considering that ER activity comes mainly from relatively large particles of O(mu m), which are mostly present in our system, Span85 was the best choice for a stabilizing additive in the present ER suspension. In addition, our results showed that the presence of the nonionic surfactants considered here did not cause any sensible degradation at high temperatures.