The genus Acarospora and the family Acarosporaceae are among the most complex and poorly known lichen-forming ascomycetes. Cryptolecanorine apothecia and multiple post-meiotic mitoses resulting in polyspory are features that characterize the family. Because many characters are cryptic or are highly variable within Acarospora, their classification at almost all levels (species to order) is very unstable. Preliminary DNA sequence data revealed that Acarospora is among the basal members of the "Lecanoromycotina". This basal phylogenetic position renders it of particular importance for understanding the evolution of the majority of lichenized ascomycetes. We will report on results from phylogenetic analyses of SSU nrDNA, LSU nrDNA, and ITS sequence data and present evidence for a re-circumscription of Acarospora. We will test for monophyly of the yellow (subgenus Xanthothallia) and brown (subgenus Phaeothallia) Acarospora and suggest approaches for achieving a new and stable classification for Acarosporaceae.

Key words: Acarospora, Acarosporaceae, phylogeny, ribosomal RNA