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 sphaerale, Strigula taylorii, Stromatopogon cladoniae and Zyzygomyces bachmannii which were previously known in Belgium from only one or two localities. Strigula taylorii is considered to be a rapidly expanding species in Belgium. Several macrolichens of great conservation value have been observed: Dermatocarpon meiophyllizum, Leptogium cyanescens, Nephroma parile, Ricasolia virens, Peltigera leucophlebia, Scytinium palmatum and Vahliella leucophaea. The majority of records of Porina aenea in Wallonia are probably related to Porina byssophila, which grows both on the bark of various trees and rocks. The status of integral natural reserve of the most diversified sites is strongly desired in order to preserve the species of conservation interest linked to the forest and rocky habitats.