The phylogenetic placement of Collema fasciculare (L.) F. H. Wigg, the most deviating species within Collema (Collemataceae, Lecanoromycetidae), was studied using maximum likelihood and Bayesian analysis of three molecular loci (mitochondrial SSU rDNA, and the protein-coding, nuclear RPB1 and MCM7 genes). Collema fasciculare belongs to Arctomiaceae (Ostropomycetidae) forming a strongly supported monophyletic group with members of Arctomia. The spores, paraphyses, asci and hymenial reactions in Collema fasciculare are similar to other Arctomia, but the ascoma ontogeny is somewhat different. Also C. leptosporum Malme, another species in the Fasciculare group, shows similarities with Arctomia regarding spores and asci. Arctomia is morphologically heterogeneous and the genus is in need of revision. Until then, the species of the Fasciculare group fit best in Arctomia s. lat. and the combinations Arctomia fascicularis (L.) Otálora & Wedin, Arctomia leptospora (Malme) Otálora & Wedin, Arctomia papuanarum (Degel.) Otálora & Wedin and Arctomia uviformis (Hue) Otálora & Wedin are proposed.
