Invasive alien plants are of increasing global concern. These plants potentially harm local biodiversity, economy, public health and road safety. Therefore, they are being increasingly counteracted. Before going into action, knowledge is needed on the distribution of invasive alien species, the desirability of their eradication and control methods. In accordance with the Upper Scheldt Department of Waterwegen en Zeekanaal NV (W&Z), INBO has performed an exploratory screening of the distribution of invasieve alien plants along navigable watercourses in West- and East-Flanders (Belgium). This is a first attempt to assess the problem of invasive alien species along these watercourses, in order to underpin options for control. In 2011 ca. 1250 kilometers were investigated for the presence of invasive alien plants, as defined by the ISEIA-protocol. 23 invasive alien plant species were recorded, of which 13 belong to the ‘black list’ (species with the highest biological impact). The top 5 consisted of Impatiens glandulifera, Fallopia japonica, Senecio inaequidens, Robinia pseudoacacia and Heracleum mantegazzianum. The presence of invasive alien plants spanned 74 kilometers of river bank (not taking overlap between species into account), 48 kilometers of which were occupied by black-list species. The distribution of invasive alien plants is not uniform. Three of the seven investigated districts hold significantly more invasieve alien plants: Ghent, Upper Scheldt, and Dender and Moervaart. The lowest presence of invasive alien plants was found along the Yser. Moreover, large differences were found between the districts regarding the presence of species. E.g., Fallopa japonica and Impatiens glandulifera were prominent in the District of Ghent, while Heracleum mantegazzianum was most common in the Upper Scheldt district. Based on literature, a table is presented in which the degree of potential nuisance is listed for each observed species. This table supports the decision which species one may want to control. ...