Xylaria hypoxylon, the type species of Xylaria (Xylariaceae, Sordariomycetes), was first described by Linnaeus as Clavaria hypoxylon from Sweden. Saccardo and other mycologists assumed a cosmopolitan distribution for this species. However, contradictory reports in the literature on its morphoanatomical characters and strongly inconsistent rDNA sequence data attributed to this species in GenBank suggested the existence of an unresolved species complex. To address this lack of clarity, molecular and morphological characters of numerous specimens and corresponding cultures of X. hypoxylon and related taxa from Europe were studied. Newly obtained 5.8S/ITS nrDNA sequence data were compared with published data and sequences of reference strains. European populations of X. hypoxylon from various hosts exhibited consistent rDNA sequence data and a relatively uniform holomorphic morphology, except for one specimen from Sweden that deviated in its ascospore morphology. Some samples from western United States showed DNA sequence data being identical to those of specimens from Europe, confirming a North American occurrence of X. hypoxylon. DNA sequences and morphology of other extra-European material however showed substantial deviations. Definitely not conspecific with the European material examined in this study is a strain, assigned to X. hypoxylon, the DNA sequence data of which have been used in various phylogenetic studies as a representative of Xylariaceae and Xylariales respectively. This material probably represents X longiana.