PURPOSE: To obtain an electrorheological fluid reducing current density and excellent in yield static stress by adding a finely divided solid suspension to an insulating fluid containing a polysiloxane having a specific polysiloxane unit.

CONSTITUTION: An electrorheological fluid is a suspension of a finely divided solid in an insulating fluid containing a polysiloxane having at least one siloxane unit having a fluorohexylalkyl group to which silicon is bonded. The pref. polysiloxane to be used has high density balanced with the density (that is, density exceeding 1.39/ml) of the finely divided solid and low viscosity (that is, below about 100 cSt at 25°C). As the pref. polysiloxane, the one represented by formula I (wherein R is a 1-6C alkyl; R' is a group represented by formula II; (x) is 0-0.5; (y) is 1-x and (n) is 3-10) is designated.