Abstract: In a hydrocyclone the separation chamber (2) has a circumferential wall (3) provided with at least one turbulence creating member (12), which extends along the circumferential wall and crosses a helical path (10) along which a liquid stream is generated during operation. According to the invention, the turbulence creating member is formed by an offset (12) on the circumferential wall (3). The offset is formed and dimensioned such that said liquid stream substantially loses its contact with the circumferential wall as the liquid stream passes the offset. As a result turbulence is created in a layer of the liquid stream situated closest to the circumferential wall, without the liquid stream developing any substantial flow component directed inward in the separation chamber.