Electrorheological fluid compositions of this invention are dispersions in an electrically insulating medium of 10 to 50% by volume of spherical silica particles prepared by hydrolyzing a silicon alkoxide of the general formula Si(OR).sub.4 in which R is an alkyl group in the presence of an alkali catalyst and drying at or below 500.degree. C., show excellent fluidity in the absence of an applied voltage, vary reversibly to the state of high viscosity or even to the state of gel on application of a voltage, and have good storage stability.