Protected areas and the Natura 2000 network are keystones of the EU nature and biodiversity policy. However, alien plants do not stop their spread at the border of protected areas and invasive plants are reported to threaten many ecosystems, notably in aquatic and riparian areas. A stratified sampling of the Natura 2000 network in Wallonia revealed that at least 41 alien plant species were present. 75 % of the sampled sites were invaded by at least one alien plant species.