The lichens and lichenicolous fungi of the Semois valley between Bouillon and Bohan, with nineteen species new to Belgium. 335 species of lichens and 24 species of lichenicolous fungi were recorded during inventories carried out between 2020 and 2023 in the section of the Semois valley between Bouillon and Bohan. Nineteen species are new to Belgium: Abrothallus caerulescens , Arthonia digitatae , A. graphidicola , Bacidina piceae , Chaenothecopsis ochroleuca , Cresponea premnea , Dirina fallax , Lecanora cenisia , Lichenostigma alpinum , Mycoblastus caesius , Porina collina , Placopyrenium breussii , Polycoccum microsticticum , Ramonia chrysophaea , Rhizocarpon postumum , Rimularia intercedens , Synarthonia leproidica , Verrucaria devensis and V. lapidicola . New localities were discovered for Aquacidia antricola , Arthopyrenia salicis , Coniocarpon cuspidans , Eopyrenula grandicula , Opegrapha areniseda , Porocyphus coccodes , Pronectria pertusariicola , Rhizocarpon infernulum f. sylvaticum , Sclerococcum ...