Ten barley mutants and one barley line, each with an independently arisen resistance gene in the ml-o locus, and some susceptible cultivars were inoculated with Erysiphe graminis f. sp. hordei conidia suspended in a fluorochemical liquid. The germination percentage of the conidia was not affected by the hosts. An average of about 60% of the germinated conidia formed haustoria, 50% formed hyphae, and 40% formed mildew colonies on the susceptible hosts. In contrast, only about 0.7% formed haustoria, 0.4% formed hyphae, and 0.2% formed colonies on the resistant hosts, on which the great majority of haustoria were located in the subsidiary cells next to the guard cells. Secondary haustoria were formed in susceptible hosts by 84% of the conidia that formed primary haustoria. In resistant hosts, the percentage was as high as 36, and they were almost exclusively formed in epidermal cells adjacent to invaded subsidiary cells. This suggests that the adjacent epidermal cells become less resistant, allowing colony development. The frequency of colonies differed significantly among the resistant barleys; this could be ascribed to differences in the genetic background. Pathogen cultures derived from the occasional colonies on the resistant barleys were avirulent on ml-o resistant barleys. The 11 ml-o resistance genes all affected the primary infection and the formation of mildew colonies, equally.