In the years 1997–2011, studies were conducted to assess the persistence and stability of the share of Lolium perenne cultivars (Polish varieties Anna, Arka, Maja, Solen, Rela and the SZD-291 strain, and Dutch varieties Baristra, Barezane and Barlano) in the sward of two grassland mixtures. The experiments were established on peat-muck soil (Mt II). The greatest impact on stability of the share of the particular cultivars of Lolium perenne in the sward of the tested mixtures was exerted by unfavourable thermal conditions of the winter of 2002/2003, high or low precipitation volumes, and the related varying groundwater level in the second half of the growing season preceding the assessment of the species composition.