Floristical investigations were carried out in 2009 in both terrestrial and littoral zones along the NW shore of the mesotrophic Lake Powidzkie (a Natura 2000 site: PLH300026). The results comprise: a general comparison of aquatic vs. terrestrial species richness; a census of 296 species of vascular plants and 7 species of charophytes; the taxonomical, biological (life forms) and geographical-historical (native vs. alien taxa) structure of vascular flora. Distribution of nationally and regionally threatened and legally protected species is presented on maps which illustrate that most of these taxa are concentrated in Lake Powidzkie and in its neighbourhood. The ongoing synanthropization of the flora is assessed and discussed considering localities of chosen alien species and their occurrence within natural plant communities: 21 of 27 aliens were recorded at least once in natural vegetation, whereas 4 of them (Aster lanceolatus, Elodea canadensis, Impatiens parviflora and Rhus typhina) formed their own, xenospontaneous communities. The main conclusion is that while the area still abounds in regionally valuable components of native biodiversity, it simultaneously is more and more threatened by increasing anthropopressure, which has already been manifested by the presence of many potentially invasive alien species.