The infection process of Colletotrichum orbiculare on the leaves of susceptible and resistant cucumber cultivars were studied histologically. On both cultivars, conidia began to germinate 8 h after inoculation (hai) and formed appressoria. Then melanised appressoria formed penetration pegs at 10 hai. On the susceptible cultivar, infection vesicles formed within 24 hai and developed thick, knotted primary hyphae at 48 hai. By 72 hai, C. orbiculare produced highly branched secondary hyphae that invaded underlying mesophyll cells. While on the resistant cultivar, fewer germinated conidia developed biotrophic primary and invasive necrotrophic secondary hyphae than on the susceptible cultivar. These results show that the early stages of penetration process on both cultivars have no differences, and that resistance in cucumber restricts colonization by inhibiting the development of biotrophic primary hyphae and necrotrophic secondary hyphae.