We present a molecular phylogenetic analysis for two families within the Pleosporomycetidae (Dothideomycetes), the Hysteriaceae, and the Mytilinidiaceae, using four nuclear genes, the ribosomal LSU and SSU, transcription elongation factor 1 α and the second largest RNA polymerase II subunit. Multigene phylogenies provide strong support for the monophyly of the Hysteriaceae and of the Mytilinidiaceae, both within the Pleosporomycetidae. However, sequence data also indicate that both families are not closely related within the subclass. Although core groups for many of the genera in the Hysteriaceae have been defined, Hysterium, Gloniopsis, and Hysterographium are polyphyletic, with affinities not premised on spore septation and pigmentation. Glonium is also polyphyletic, but along two highly divergent lines. The genus lies outside of the Hysteriaceae, and finds close affinities instead with the family Mytilinidiaceae, for which we propose Gloniaceae fam. nov. to accommodate the type, G. stellatum and related forms. The genus Psiloglonium is reinstated within the Hysteriaceae, with P. lineare, as type, to accommodate non-subiculate species, with apically obtuse didymospores. Farlowiella is removed from the Hysteriaceae, but remains within the Pleosporomycetidae. In contrast, despite divergent spore morphologies, the genera Mytilinidion and Lophium form a strongly supported clade, thus defining a highly monophyletic Mytilinidiaceae, adjacent to the Gloniaceae, for which we propose the Mytilinidiales ord. nov. The genus Ostreichnion, previously in the Mytilinidiaceae, is here transferred to the Hysteriaceae. It is concluded that the evolution of the hysterothecium occurred multiple times within the Pleosporomycetidae, and alone it is not a synapomorphic character state for the Hysteriaceae.