Abstract: In a hydrocyclone separator for separating a mixture into one light and one heavy fraction, with a separation chamber consisting partly of one circular cylindrical part and partly of one conical part, the wall of the separation chamber (1, 2) is provided, along a substantial part of its axial elongation, with a means creating a turbulence to a relatively moderate degree and which extends in a screw path (7) opposite to the screw path in which the flow streams along the wall of the separation chamber. Advantageously, at least one guide bar (6) with substantially axial elongation is provided to give the flow, streaming in a screw path along the wall of the separation chamber (1, 2) a radially inwardly directed component. The screw-path-formed limiting surface (7) of said means may form the angle .alpha.=0.degree.-75.degree. to the axial symmetry axis (9) of the separation chamber (1, 2).