Filling material for percolating filters used in the biological treatment of fluids are provided with strips which are made of plastic and are connected to one another via a transversely directed holding web. The strips are arranged one above the other in a plurality of layers in order to increase the strip density. In the filling material according to the invention, the strips are provided with different cross-sections over their length. For example, the strips may be reduced in width in regions adjacent to their connection to the holding web. As a result, the strips can be arranged in one layer with an increased strip density.