The sequence of events leading to infection by Discula umbrinella of leaves taken from axenically grown beech seedlings was studied at the ultrastructural level. On the host surface, even under optimal conditions and when contact with the host is established, only approximately 70% germination can be observed. In hanging drops containing malt extract, germination is only approximately 10%, suggesting that abundant nutrient supply alone is not enough to promote germination. Incubation of conidia in leaf extracts, on the other hand, results in the induction of germination even if no contact with an appropriate support is established. Between 16 and 24 h after infection four stages of germination can be observed on beech leaves: (i) ungerminated conidia; (ii) germinating conidia producing long superficial hyphae; (iii) conidia with distinct, appressorium-like swelling at the end of the germ tube; and (iv) germinated conidia with a halo around the tip of the germ tube. Transmission electron microscopy shows that subcuticular penetration into the host occurs at places where halos are seen. Apparently only germination with formation of appressorium-like structures and halos leads to successful penetration. Application of conidia to the abaxial side of the host leaf results in the formation of structures similar to those observed on the adaxial surface. Conidial germ tubes penetrate the cuticular layers of the outer edges of the guard cells, but direct penetration through the stomata was rarely observed. Appressorium or halo formation, observed only on the host surface and never on other substrates, suggests host-specific induction of penetrating structures. The mode of penetration observed for D. umbrinella bears some similarities with that described for some biotrophs and pathogens, suggesting affinities between endophytes and pathogens.