The lichenicolous fungus “Pleospora” aquatica is described as new. It grows parasitically on thalli of Aspicilia supertegens, and is known only from the type locality, in the Tunkinski ridge (central Siberia). Is is tentatively referred to the genus Pleospora on account of its phaeodictyospores and of the anatomy of pseudothecia. Key-words: Dacampia I lichenicolous fungi I Pleospora I Pleosporaceae I systematice.