Hydrophobins are proteins produced by filamentous fungi that exhibit very peculiar properties. Their behaviour in solution resembles that of typical surfactants characterized by self-assembling into an amphipilic membrane. Actually the hydrophobin self-assembly at the cell-wall–air interface plays a key role in the formation of aerial hyphae and fruiting bodies by establishing a waterrepellent layer. The interest in this class of proteins is increasing because of their multiple and attractive potential applications. Just to give an example, hydrophobins can be used to stabilize emulsions, and this property might be extremely useful for pharmaceutical applications and in the food industry. We have purified and characterized the hydrophobin that is secreted in the cultural broth by the mycelia of the edible fungus Pleurotus ostreatus. Higher amounts of the same protein have been obtained from the fungal mycelia and by recombinant expression in E. coli. In this paper the emulsifying properties of the protein are also discussed.