In November 1997 powdery mildew of cyclamen was found in Nagano prefecture, Japan. White, powdery mycelial colonies appeared on petals. Artificial inoculation with the fungus was successful on cyclamen petals, but not on primula or begonia plants. Conidia were cylindric to doliform, 31-51×14-20μm, formed singly on conidiophores erected on aerial mycelium. Base of the conidiophores were almost straight but sometimes slightly curved. Lobed appressoria differentiated on germ tubes from conidia. No cleistothecium was observed. On the basis of the conidial stage, the fungus was identified as an Oidium sp. of the Erysiphe polygoni type.