The Natura 2000 network consists of sites designated by the member States of the European Union, under the Habitats and Birds Directives. Setting up that network is one of the biggest challenge in nature conservation in Europe, since habitats and species for which Natura 2000 sites are designated must be maintained in a “favorable conservation status”. Little is known so far, however, about how Natura 2000 sites are invaded by exotics species. Xeric habitats of high biological value included in the Natura 2000 network are among the most species-rich in Southern Belgium. They include calcareous grasslands, sandy meadows, dry heathlands, boxwood stands, siliceous rocks and calcareous rocks. We randomly sampled 15% of sites in each of these six categories (with a minimum of five sites per category) , with a total of 86 sites out of 470 existing sites. In each site, we recorded the presence/absence of 63 alien plants know to develop in xeric habitats (species list based on Verloove (2006) and expert’s personnal observations.), and estimated species cover. 25 naturalized alien plant species were observed in xeric habitats of the Natura 2000 network in Southern Belgium. Generally, alien species populations were still limited.