During the past 15 years the saprophytic fungus Aspergillus fumigatus has become the most prevalent airborne fungal pathogen, causing severe and often fatal infections especially in immuno-compromised patients. Germination of inhaled conidia is an early and crucial event in the infection process of A. fumigatus. In this study we have analyzed morphological changes that take place during this differentiation process using scanning electron microscopy. Our data show that (i) the hydrophobic surface layer of resting conidia seems to be shed before the cells start to swell and (ii) that filamentous surface appendages are expressed at a very early phase of the germtube formation. These surface structures were only found on the first few Wm of the germtube, but were absent from the surface of mycelial hyphae and resting or swollen conidia. The highly regulated expression of these novel surface organelles suggests that they may play an important role during early germination and represent a potential target for future anti-A. fumigatus therapies. ß