Bacidia rosellizans S. Ekman is described as new to science. Morphologically, the new species is similar to the type species of the genus, B. rosella (Pers.) De Not., particularly in the pale pink and pigment-deficient apothecia and in having the proper exciple and upper part of the hymenium inspersed with minute crystals that consist at least partly of atranorin. Bacidia rosellizans, however, differs in having a thin whitish thallus, smaller apothecia and a thinner apothecial margin, a dense layer of crystals along the excipular rim, shorter and narrower ascospores with fewer septa, and septate conidia. Whereas B. rosella is a species occurring in the nemoral zone of Europe and possibly northernmost Africa and parts of Asia (but not North America), B. rosellizans is found mainly on Populus and Salix in taiga. This species is currently known from Sweden, Russia, Canada and the USA. It was erroneously treated as Bacidia rosella in a monograph of North American corticolous species of Bacidia and Bacidina by Ekman (in Opera Botanica 127, 1996).
