A culture of cooksfoot powdery mildew fungus (Erysiphe graminis DC.; culture D1) was isolated at Tokyo in 1985. No cleistothecium was observed in this fungus as previously described in Japan. The results of inoculation tests to 11 species from 8 genera of gramineous plants revealed that the host range of the fungus was limited to Dactylis sp.. This fungus was identified as a new type of forma specialis, E. graminis f. sp. dactylidis.