Abstract: A cyclone separator for separating substances from a fibre-liquid-suspension, particularly a paper pulp suspension, wherein the separator has a main part (3) which includes an inner conical chamber (4) that has a generally tangentially directed inlet at the wider end (5) of the chamber, and in which the chamber (4) tapers conically downwards. The inner surface of the chamber has a screw-like configuration characterized by a helix. The screw-like inner surface includes a first upper surface or flank (1) facing towards the wider end of the chamber and a second, lower surface or flank (2) facing towards the narrower end (6) of the chamber. The flank (1) defines an angle .alpha. with the chamber center line and the second flank (2) defines an angle .gamma. with the first flank (1).