The infection process of Sphaerotheca fuliginea into epidermal cells of host cucumber and nonhost wheat were studied histochemically. The percentages of germinated conidia had no significant difference on cucumber and wheat leaves at the pre-penetration stage. When S. fuliginea overcame the constitutive barriers of wheat leaf, papillae and hypersensitive reaction were induced to prevent the further development of S. fuliginea. Less than 1% haustoria could form in wheat at the post-penetration stage. However, in cucumber to S. fuliginea interaction, plentiful secondary hyphae and haustoria formed in cucumber leaves.