Abstract: A method of breaking up bundles of adherent hard fibers, particularly steelfibers and feeding the obtained single fibers to a mixing device. The method comprises vibrating said bundles of adhering fibers on an oscillating screen to obtain separated fibers, passing said separated fibers being aligned in a plane parallel to the plane of said screen through oblong openings of said screen, through which openings in the main only one fiber at a time can pass and supplying by means of gravity and oscillating force said individual fibers to said mixing device spread over a large area, said area being larger than the area of said screen. The screen (4) has oblong openings (8) having a length corresponding mainly to the length of the fibers and a width being essentially less than the length of said fibers.