A chemical and morphological alteration of the epidermal wall of barley, clover, strawberry, and sunflower occurred around the infection peg of Erysiphe graminis, E. polygoni, Sphaerotheca macularis, and Erysiphe cichoracearum respectively while the peg was passing through the epidermal wall. A disc-shaped area with the peg as the central point always stained more heavily than other zones after epidermal strips were treated with azure dyes and cotton blue. The "classic" zinc–chlor–iodide and the IKI–H2SO4 cellulose tests indicated that cellulose was lacking in the disc-shaped zone. This zone stained only faintly with the periodic acid – Schiff's test, indicating that the amount of polysaccharide was reduced around the penetration tube. After differential extraction of polysaccharides the PAS-negative halo was always surrounded by a PAS-positive band even after pectin, hemicellulose, and noncellulosic polysaccharides had been removed. Electron micrographs showed that the morphology was altered and electron density was increased in the disc-shaped zone. Sometimes the altered zone was swollen. Thus, the cytochemical tests and electron micrographs suggest very strongly that the cellulose wall in the zone around the infection peg is degraded by enzyme(s), such as cellulase, produced by powdery mildews.