The objective of this research project was the estimation of the range of variability of biological properties of Arrhenatherum elatius affecting its nutritive value and reproductive potentials. Investigations were carried out in years 1996-1997 on plant material derived from the collection of Arrhenatherum elatius ecotypes. The evaluation of ecotypes took into consideration such traits as: tussock formation, size and morphological structure of generative shoots, assimilation area of leaf blades, capability of developing generative and vegetative shoots, plant vitality and date of heading. The above-mentioned investigations correspond with research results of other authors who undertake investigations on the biological variability of such species as: Alopecurus pratensis (GOLIŃSKA, 1999), Festuca protensis (SAWICKI, 1999), Dactylis glomerata (SCHMIDT, 1987) and others (KOZŁOWSKI, 1997). Within the examined ecotypes of Arrhenatherum elatius, a considerable variability of the following analysed biological properties. It can, therefore. be said that there are possibilities of undertaking breeding work with the aim to improve nutritive value and increase the reproductive potentials of cultivars of this species and consequently, boost up the importance of tall oat-grass as a fodder plant. The main impetus in breeding work should be directed towards the improvement of the morphological structure of generative shoots, especially the enlargement in the share of leaf blades and plant assimilation area. The application of assessment criteria with reference of the collected ecotypes allowed indicating the best materials to be used in breeding work.