Barley plants experimentally infected with Erysiphe graminis f. sp. hordei were subjected to a curative treatment with tetraconazole (TC), to find out if the thickenings induced by the treatment in the fungal and host cell walls might be related to an increased production of chitosan. The presence of chitosan was assessed by colorimetric method, histological staining and ultrastructural localization with chitosanase bound to colloidal gold. The results showed that after TC treatment the amount of chitosan increase in the cell walls of hyphae and haustoria, and also in the haustorial encasements.