Abstract: Apparatus for separating fibers in open-end spinning units having a fiber separating cylinder in a cavity of a housing in which on a cylindrical surface there is provided a cleaning aperture with an impurity separating duct which latter opens by its impurity rebounding wall into a cylindrical impurity collecting chamber communicating, on the one hand, with an air supply aperture and, on the other hand, with an impurity sucking-off duct. In accordance with the invention at least one of the walls of the impurity sucking-off duct tangentially merges into the cylindrical wall of the cylindrical impurity collecting chamber, and the height of said sucking-off duct in the region of joining said collecting chamber is the same as or larger than the height of the outlet from said impurity separating duct into said collecting chamber.