Little is known about the potential effect of climate warming on phyllosphere fungi, despite their important impact on the dynamics and diversity of plant communities. The structure of phyllosphere fungal assemblages along elevation gradients may provide information about this potential effect, because elevation gradients correspond to temperature gradients over short geographic distances. We thus investigated variations in the composition of fungal assemblages inhabiting the phyllosphere of European beech (Fagus sylvatica) at four sites over a gradient of 1000 m of elevation in the French Pyrenees Mountains, by using tag-encoded 454 pyrosequencing. Our results show that the composition of fungal assemblages varied significantly between elevation sites, in terms of both the relative abundance and the presenceabsence of species, and that the variations in assemblage composition were well correlated with variations in the average temperatures. Our results therefore suggest that climate warming might alter both the incidence and the abundance of phyllosphere fungal species, including potential pathogens. For example, Mycosphaerella punctiformis, a causal agent of leaf spots, showed decreasing abundance with elevation and might therefore shift to higher elevations in response to warming.