Abstract: The invention relates to a device for forming fibre balls out of elongated fibres carried in an air flow. The device contains a stator chamber (1) which is fixed to a frame and which has a circular cross section, and coaxially within it there is arranged a rotor (2), also having a circular cross section, and being rotatable around its shaft (3), whereby the rotor has on its periphery a plurality of radial blades (12, 12′) at mutual peripheral distances and at a short distance from the inner wall (1′) of the stator (1) in order to form an annular intermediate space (14) extending over the axial length of the rotor between the inner wall (1′) of the stator (1) and the blades (12, 12′) of the rotor (2).