Phylogenetic relationships of the fungal symbiont of two brown algae were evaluated in the context of host diversity using ribosomal large subunit (nuLSU) sequences. Phylogenetic analysis indicates that Mycophycias ascophylli (phycobionts Ascophyllum nodosum, Pelvetia canaliculata) is misclassified in Verrucariales (Eurotiomycetes). There was no evidence for speciation of M. ascophylli within its two hosts, nor evidence for geographic differentiation between the eastern and western Atlantic Ocean and the White Sea (Arctic Ocean). The placement of M. ascophylli in Capnodiales (Dothideomycetes) will require a reclassification of this species, and calls into question the current classification of the other species of Mycophycias, M. apophlaea. © 2011 by Walter de Gruyter Berlin Boston.