Abstract: The endophytism of Acremonium byssoides in Vitis vinifera was recently ascertained in Sicily. In particular, the hyphomycete was observed in leaves of three vine cultivars (Regina Bianca, Catarratto and Insolia). Moreover, in the leaves of cultivar Insolia inoculated with P. viticola, the A. byssoides showed an antagonistic activity (hyperparassitism and antibiosis) towards asexual and sexual structures of the oomycete.In spring 2002 and 2007 “Insolia” vines, infected by the endophyte, suffered repeated attacks by P. viticola that lasted until the formation of gamic structures of pathogen. The aim of our researches was to ascertain in vivo the effect of A. byssoides on viability of oospores, the only means of P. viticola overwintering. The “mosaic spotted” leaves were collected in October from vines colonized (cv. Insolia) or not (cv. Catarratto) by A. byssoides and exposed to naturalclimatic conditions. The oospores viability was assayed by germination tests.The differentiated oospores in endophyte-free leaves showed the highest mean germination value, whereas the other ones were degenerated and did not germinate at all.This study shows that the interaction between A. byssoides, V. vinifera and P. viticola could assume a determinant role to contain the mildew infections in our environment.