Technical Abstract: The taxonomy of the powdery mildew fungus infecting lentil in the Pacific Northwest (PNW) of the USA was investigated on the basis of morphology and rDNA ITS sequences. Anamorphic characters were in close agreement with descriptions of E. trifolii. However, teleomorphs formed chasmothecial appendages with highly branched apices, whereas E. trifolii has been described as producing flexuous or sometimes loosely-branched appendages. Branched appendages have been described in E. diffusa, a fungus reported from species of Lens, Glycine and Sophora, raising the possibility that the PNW fungus could be E. diffusa. Examination of an authentic specimen of E. trifolii from Austria determined that it included chasmothecial appendages resembling those seen in PNW specimens. Furthermore, ITS sequences from five powdery mildew samples collected from lentils in PNW greenhouses and fields from 2006 to 2008 were identical to one another, and exhibited higher similarity to sequences of E. trifolii (99%) than to those of any other Erysiphe spp. available in GenBank. Parsimony analysis grouped the lentil powdery mildew into a clade with E. baeumleri, E. trifolii and E. trifolii-like Oidium sp., but indicated a more distant relationship to E. diffusa. In greenhouse inoculation studies the lentil powdery mildew fungus did not infect soybean genotypes known to be susceptible to E. diffusa. The pathogenicity of E. trifolii on lentil was confirmed using modified Koch’s postulates. This is the first report of E. trifolii infecting lentil. Erysiphe diffusa and E. trifolii occur on different host ranges, so the discovery of E. trifolii on lentil has implications both for determining species of powdery mildews on cool season grain legumes and also in disease management.