The article presents the results of a lichenological study in the forest communities of the northern part of the Kostomukshsky State Nature Reserve, carried out in June 2017. The species diversity of lichens was studied on all substrates within 25x25 m sample plots.

The total survey area was 1 ha and 950 samples of lichens were collected. As a result, 143 species of lichens were identified. Information about newly detected species is presented in the annotated list, which includes 68 species of lichens and allied fungi new to the Kostomukshsky Reserve (25 % of the total amount): 60 species and 1 subspecies of lichens, 4 species of lichenicolous fungi, 3 species of non-lichenized fungi. The records include 43 species reported for the first time for the biogeographic province of Karelia pomorica occidentalis, 1 species (Chaenotheca subroscida) is listed in the Red Data Book of the Republic of Karelia (2020). For the species Protothelenella corrosa, the exact locality in the Republic of Karelia is given for the first time. The species Lecidea lithophila, Lobothallia recedens, Stereocaulon evolutum used to be known in Karelia only from the collections of Finnish researchers of the late 19th and early 20th centuries. Nowadays, 278 species of lichens and allied fungi have been recorded for the territory of the Kostomukshsky State Nature Reserve. Considering the territory remaining unexplored, it can be assumed that no more than 50-60 % of the species diversity of lichens and allied fungi has been revealed.