The Marsh Fern (Thelypteris palustris) is a relict species of fern associated with wet habitats, such as river banks, lakes, peat bogs, wet forests, and meadows. The largest population of Marsh Fern in Serbia has been recorded in the Special Nature Reserve Kraljevac. Here the species forms a relict plant community with the Grey Willow and the Common Reed Thelyptero—Phragmito—Salicetum cinereae and occurs on floating and permanently anchored islands. The Marsh Fern with its densely entangled rhizomes, together with strong roots of the Grey Willow, and dissolving plant matter form initial layers of peat, which in some parts can be half a meter deep. The Marsh Fern is a strictly protected species under the Code on Declaration and Protection of Strictly Protected Species and Protected Wild Species of Plants, Animals and Fungi Annex I (Official Gazette No. 5/10). It is also listed in the Red Book of Flora of Serbia, Vol. I.