The article presents information on the potential, "hidden" mycobiota diversity of fallen P. abies logs in advanced decay classes (III-V). The surveys were conducted in intact native bilberry spruce forests in the middle taiga subzone within the Kivach State Nature Reserve (Republic of Karelia). The object of the study is the metagenome of fallen deadwood. Data were obtained using high-throughput sequencing (next-generation sequencing, NGS). We collected 23 mixed samples from 12 fallen P. abies logs of 3-5th decay classes in September 2023. A total of 3461 metagenomic operational taxonomic units (mOTUs) of fungi were identified. Of these 80 % (2759 mOTUs) were identified to the species level according to NCBI: Ascomycota (1638), Basidiomycota (805), Blastocladiomycota (9), Chytridiomycota (56), Cryptomycota (2), Microsporidia (47), Mucoromycota (133), Olpidiomycota (2), and Zoopagomycota (67). The taxonomic structure of the mycobiota belonging to Ascomycota and Basidiomycota is represented down to the genus level. The composition of the mycobiota was characterized by high species diversity (Shannon index 5.0-6.2), high distribution uniformity (Pielou index 0.7-0.9) and low dominance measure values (Simpson index 0.004-0.2). The samples contained 12 fungal and 5 lichen species red-listed in the Republic of Karelia. The red-listed Ganoderma lucidum and 3 lichenized fungi Cladonia strepsilis, Dolichousnea longissima, and Gyalecta friesii, as well as one species in need of special attention to its status in the natural environment Gyalecta truncigena, were identified for the first time for this territory.