Abstract: A method of, and apparatus for, forming a fiber layer of substantially constant weight per unit length, there being provided supply means for supplying a fiber and air mixture onto a moving, fiber-separating, perforated surface. A suction opening is arranged beneath and extends over a certain zone or region of such perforated surface. The suction opening can be moved forward and backward with respect to the direction of transport of the fiber and air mixture by the perforated surface as a function of a measuring value which is representative of the thickness of the fiber layer deposited upon such perforated surface.