The aim of our investigations conducted on grasslands situated in the region of Wielkopolska was to analyze changes in the occurrence of selected plant species in the sward of meadow communities, to determine causes of their expansion and the impact on natural and useful values of these plant complexes. The occurrence of Deschampsia caespitosa, Holcus lanatus and Cirsium arvense from meadow communities was subject to comparative analysis. The behaviour of the above species, described as aggressive, was analyzed on the basis of changes in the frequency of their occurrence (i.e. constancy degree) in communities and the achieved ground covering coefficient. In addition, causes and consequences of the expansive behaviour of the analyzed plant species were assessed. The expansive behaviour of Deschampsia caespitosa, Holcus lanatus and Cirsium arvense is associated, on the one hand, with the occupation of new sites and, on the other, with a significant increase in the proportion of these species in the sward as evidenced by the obtained values of the ground-covering coefficient. The spreading the analyzed plant species results from the direct or indirect anthropo-pressure on sites and vegetation type of meadow communities. The discussed expansive species – course hairgrass, velvet grass and Canadian thistle – occurring in grass communities, especially during longer time intervals, exert unfavourable influence on their natural and use values.