The purpose of this paper is to determine the changes in the concentration of different fractions in the cell wall of Dactylis glomerata and Festuca pratensis growing on both mineral and organic soil with different harvesting times. This paper has drawn on two field experiments set up and carried out between 2010 and 2013 by the Research Centre for Cultivar Testing in Słupia Wielka. The experimental plots were sown with varieties of Dactylis glomerata: Niva, Tukan, Amila, Crown Royale and with varieties of Festuca pratensis: Limosa, Pasja, Anturka, Amelka (d. AND 1009). The experiment in Krzyżewo was set up on mineral soil. In Uhnin the experimental plots were located on peat meadow. The full exploitation of Dactylis glomerata varieties was due between 2012 and 2013, whereas for Festuca pratensis it was due between 2011 and 2012. In the experimental plots with the varieties of Dactylis glomerata the grass was harvested six times a year and chemical analysis of the biomass was done taking dry matter only from five cuts. The varieties of Festuca pratensis were harvested four times. Each year in the course of the experiment fresh and dry matter of each cut were weighed. The obtained results showed that the time of the harvest or cut for both of the grass species showed significantly different concentrations of the neutral detergent fiber fraction (NDF), acid detergent fiber fraction (ADF) and different concentrations of lignin and cellulose. For both of the grass species, plants harvested in the fourth cut had the highest content of different cell wall fractions. The location, that is the type of soil, affected the concentration of lignin (ADL), but only in the case of Festuca pratensis. A higher concentration of this polysaccharide was in the grass growing on organic soil.