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Quintaria microsporum sp. nov., found on submerged wood of Fraxinus excelsior in the bed of a stream in Ariège, Rimont, Peyrau, France, is described, illustrated and compared with similar taxa. It is unique in Quintaria in having small ascospores and asci. A key to species of Quintaria is provided. This is the first record of a Quintaria species from a freshwater environment in Europe.