A description is given of Pythium litorale sp. nov., a new species from reed stands in Germany. Pythium litorale was among the most abundant species when the oomycete community of littoral soils of Lake Constance was studied. It was consistently isolated from flooded as well as from drier reed sites. The species is characterized by subglobose, papillate and internally proliferating sporangia, globose hyphal swellings, the absence of oogonia in single culture and a high optimum growth temperature. It proved to be nonpathogenic to Phragmites australis, the predominating plant in the investigated sites. Molecular analysis of ribosomal DNA internal transcribed spacer regions placed Pythium litorale in a clade together with its closest relatives Pythium megacarpum, Pythium boreale, Pythium montanum and Pythium carbonicum. The generic status of this basal clade in Pythium is currently under discussion, as it possibly represents a separate genus that is distinct from Pythium, and shares several characteristics with Phytophthora.