Silicone antifoamers

An antifoamer which comprises a polyperfluoroether group-containing organopolysiloxane comprising from 20 to 100 mole % of the polyperfluoroether group-containing organosiloxane units of the following formula (1) ##STR21## wherein Q represents a divalent organic group having from 1 to 40 carbon atoms and is ##STR22## wherein R.sup.3, R.sup.4 and R.sup.5 independently represent a divalent hydrocarbon group having from 1 to 10 carbon atoms, a and b are independently an integer of from 1 to 5, p and q are independently an integer of from 1 to 10, and r and t are independently an integer of from 2 to 5, R.sup.1 is selected from the group consisting of a hydrogen atom, an unsubstituted monovalent hydrocarbon group having from 1 to 8 carbon atoms, a substituted monovalent hydrocarbon group having from 1 to 8 carbon atoms in which part or all of the hydrogen atoms bonded to the carbon atoms are substituted with a halogen atom, a cyano group, a chloromethyl group, a 3,3,3-trifluoropropyl group or a 2-cyanoethyl group, and a --OX group in which X represents a hydrogen atom or a monovalent hydrocarbon group having from 1 to 8 carbon atoms, m is an integer of from 1 to 14, n is an integer of 1 to 10, and x and y are 0<x.ltoreq.3 and 0.ltoreq.y.ltoreq.3 provided that 0<x+y.ltoreq.3, the content of fluorine atoms in the polyperfluoroether group-containing organopolysiloxane being in the range of from 20 to 70% by weight.