Multiwalled carbon nanotubes ( MWCNTs) were incorporated onto the surface of polystyrene ( PS) microspheres by a simple and potentially scalable process. PS microspheres, 3.0 mu m in diameter, were prepared using a dispersion polymerization method. These were adopted as the suspended particles for electrorheological ( ER) fluids, in which the electrical conductivity was mainly derived from the conducting MWCNTs layers coated on the surface of the microspheres. The MWCNT= PS composite suspensions in silicone oil showed the typical characteristics of an ER fluid, forming a chain- like structure under an applied electric field ( 0.8 kV= mm). The MWCNTs= PS composite microspheres exhibited a conductivity of about 2.0 x 10(-4) S/cm, which is in the typical range for ER fluids. This phenomenon can be explained by the interfacial polarizability of the MWCNTs adsorbed on the surface of the polymeric microspheres.