Acarospora badiofusca has an irregularly areolate thallus with an uninterrupted algal layer; it is a montane species and occurs in Asia, North America and Europe. Acarospora boulderensis differs from A. badiofusca in having a squamulose thallus, an interrupted algal layer and a higher hymenium. Acarospora boulderensis was considered a synonym of A. badiofusca by North American taxonomists, but is here recognized as a distinct species occurring in North America. Acarospora asperata H. Magn. is revised as a synonym of A. boulderensis. Acarospora irregularis from central Europe (Czech Republic, Hungary and Slovakia), as well as Greece and Italy (Sardinia), was considered a synonym of either A. badiofusca by Weber or of A. nitrophila by Clauzade and Roux. It differs, like A. boulderensis, from A. badiofusca in having a squamulose thallus, an interrupted algal layer and usually a higher hymenium. However, A. irregularis differs from A. boulderensis in having wider hyphal bundles interrupting the algal layer and distinct algal palisades. Acarospora badiofusca var. lepidioides is synonymized with A. irregularis.