Studies conducted in 1996–2002 aimed to evaluate the durability of species composition of meadow mixture sward with different varieties of Lolium perenne in post-bog habitat (peat- -bog soil). The research was carried out on meadows of Didactic and Scientific Station in Sosnowica (Wieprz-Krzna Canal region) in two parallel experiments (A and B). The experiments included the sowing of 11 mixtures of Polish (Anna, Arka, Maja, Rela, Solen, strain SZD-291) and Dutch (Baristra, Barezane, Barlano) varieties of perennial ryegrass. The sward was cut three times at in optimum dates for three-cut meadow cutting. On the basis of analysis of plant samples from the first cut of the following study years, changes of sward species composition of the sown meadow mixtures were evaluated. Basic species tested – perennial ryegrass – was characterized by quite stable share in the sward of mixtures with a tendency to decrease in the last years of the experiments. High differentiation of the species durability depending on the variety was found. The following varieties were distinguished with higher share stability and durability: Barlano, Maja, Baristra, Anna, Solen and strain SZD-291 (as well as a mixture of Polish varieties), in opposition to: Rela, Arka and Barezane cv., whose share in the sward decreased up to 1.5–8.0% in the last year of the experiment. The results point out that perennial ryegrass may be the species of grass communities reducing the increase of aggressive bluegrass share. Papilionaceous plants that almost completely vanished after three years of performance were characterized with the lowest durability in the sward of sown mixtures. Timothy grass that reduced its share in the following years of performance showed relatively low durability in contrast to cocksfoot and reed fescue.