Abstract The authors present the results of the lichenological inventory of the natural site of the Cascades of Mortain in the department of Manche. The lichen vegetation is briefly described; a list of 263 taxa is given, from which are extracted 24 species of interest in French flora. Anisomeridium biforme (Borrer) RC Harris (not a very common species in France), Arctomia incurva (Pers.) Hale (fairly rare species in France), Arthonia graphidicola Coppins (fifth French site), Arthothelium ruanum (Massal.) Körb. (rare in France), Bactrospora patellarioides (Nyl .) Almq. (rare in France), Buellia griseovirens (Turner and Borrer ex Sm.) Almb. (fairly common in the mountains), Chaenotheca phaeocephala (Turner) Th Fr (rare at hill level), Cresponea premna (Ach.) Egea and Torrente var. premna (not very common in France), Enterographa elaborata (Lyell ex Leight.) Coppins & P. James (rare in France), Enterographa hutchinsiae (Leighton) Massal. (rare in France), Hydropunctaria rheitrophila (Zsch.) Keller, and Gueidan Thus (fairly rare), Lecania hyalina (Fr.) R. Sant. (not very common in France), Lecanora jamesii JR Laundon (rare in France), Mycoblastus caesius (Coppins & P. James) Tønsberg (known only in Lower Normandy), Opegrapha ochrocheila Nyl. (known in three French departments), Placynthium flabellosum ( Tuck.) Zahlbr. (distribution in France very poorly understood because of confusion with Placynthium rosulans (Th Fr) Zahlbr.) Porina ahlesiana (Körb.) Zahlbr. (very rare in France), Porina guentheri (Flot.) Zahlbr. (rare in France), Punctelia Reddenda (Stirt.) Krog (very rare in France), Pycnothelia papillaria (Ehrh.) Duf. (not common in France), Pyrenula chlorospila (Nyl.) Arnold (fairly rare), Pyrrhospora cinnabarina (Sommerf. ) Choisy (known in four French departments), Ramonia subsphaeroides (Tav.) Vezda (rare, new to northern France), Strigula ziziphi (A. Massal.) Cl Roux and Serusi. (new to northern France). The authors highlight the risks of environmental damage to the wealth of lichens, suggest some avenues for preservation of their environment, and finally highlight the uniqueness, in the eyes of other French regions, of the site as a plain.