A powdery mildew was observed on bean plants (Phaseolus vulgaris L.), different cvs., grown in the greenhouse of the Plant Pathol. Dept., Fac. of Agric., Cairo Univ., Giza governorate. Symptoms of the disease were noticed, primarily, on the stems of bean plants as small pale yellow spots, then enlarged and becoming purple to dark brown or rusty lesions. The affected spots became covered by white, superficial powdery growth (Fig. 1). Later, powdery mildew colonies were found on the stem branches and leaves petiole. Due to the harmful effects of infection pre-mature defoliation, plant weakness and lowering plant vigor were occurred. In most cases, severe infection on the stem can girdle it causing plant collapse. Microscopic examinations revealed that conidiophores are erect, perpendicular to hyphae, delimited by septum at the bottom of the base cell, followed by 2 to 3 straight cells. Conidia produced in chains (Fig. 2), ellipsoid, ovoid or cylindrical with fibrosis bodies, measure 23.43-28.9 x 12.48-17.5 μm. The teleomorph stage (Ascocarp) was not observed. Morphological characters are typical of catenate:Oidium anamorph (sub-genus:Eu-Oidium Jaz.). Evidence suggests that this fungus may be the Oidium stage of Sphaerothecafuliginae (Schlecht.) Poll. However, identification of the causal agent needs further studies. As the authors are far aware, this is the first record of powdery mildew on beans caused by Oidium sp. in Egypt.