The Rostania occultata species complex (‘Collema occultatum s. lat.’) is revised in Fennoscandia and found to consist of four species, all epiphytes on deciduous trees: Rostania effusa A. Košuth., M. Westb. & Wedin sp. nov., R. occultata (Bagl.) Otálora et al., R. pallida A. Košuth., M. Westb. & Wedin sp. nov. and R. populina (Th. Fr.) A. Košuth., M. Westb. & Wedin comb. nov. Rostania effusa and R. pallida are newly described from humid habitats in old-growth boreal coniferous forests, usually with a mixture of deciduous trees, and from similar areas in the subalpine birch-dominated forests of Fennoscandia. Rostania effusa is characterized by apothecia with red-brown apothecium discs and an excipulum thallinum with a simple pseudocortex and cubic to oblong, muriform spores. Rostania pallida has apothecia with whitish to pale yellowish discs and an excipulum thallinum with a distinct cellular pseudocortex, and ellipsoid, muriform mature spores that are often constricted at the centre. A lectotype is designated for Collema quadratum J. Lahm ex Körb. The new combination Rostania populina is introduced for the species recognized until now as the variety Rostania occultata var. populina (Th. Fr.) Perlmutter & Rivas Plata. A key to the six species in Rostania s. str. is included. cyanolichens, morphology, new species, Peltigerales, Scandinavia, systematics, taxonomy