Phenotypic stability parameters of traits for five perennial ryegrass cultivars were estimated on the basis of individual plant observations in two series of 3-year experiments. Among tested cultivars Argona showed remarkable stability of heading, flowering and inflorescence number per plant determined by means of ecovalence and random deviation from regression. Conversely, cv. Niga was characterised by the lowest stability of these traits and simultaneously a good stability of seed yield and adaptability of inflorescence number per plant. The parameter of ecovalence give the largest differentiation of evaluated traits. Correlation coefficient between stability parameters were computed and discussed.