An arrangement for ascertaining the presence of foreign bodies in a mass of fiber tufts includes a fiber tuft-supporting surface having a width; a device for providing a loose fiber tuft layer on the fiber tuft-supporting surface; and a foreign body detecting apparatus arranged for scanning the fiber tuft layer for foreign bodies. The foreign body detecting apparatus has a detector device formed of a plurality of individual sensors arranged side-by-side along the width of the surface. There is further provided a device for effecting relative displacement between the fiber tuft layer and the apparatus in a direction perpendicular to the surface width.