The aim of the study was to determine the biomass of some grass species, in the third year after sowing as affected by the sowing date and soil moisture. A special attention was paid to plant mass formation in the stubble and sward layer, which ensures permanence of species. The study was carried out at the Experimental Station of SGGW Grassland Division at Jaktorów. The experiment was designed in two sites: moderately wet and moderately dry. There were two sowing dates: spring (24 April) and late-summer (3 September). The tested grass species were: L. perenne (cv. Argona), F. pratensis (cv. Skra), D. glomerata (cv. Baza), B. inermis (cv. Brudzynska) oraz A. elatius (cv. Wiwena). At the time of cuts plants were taken from 30 cm of row sector, width of 12 cm and at a depth of 30 cm. The aboveground biomass (above 5 cm – yield), mass of stubble (0–5 cm above ground) together with sward (0–5 cm below ground) and the mass of roots were determined. Site moisture and the sowing period are of great importance in biomass distribution in particular layers. Irrespective of the sowing period, plants form greater aboveground biomass in a moderately wet site than in a moderately dry one, while in a moderately dry site – greater biomass in the sward and stubble layer and roots, especially in II and III regrowths. The contribution of the biomass in the sward and stubble layer to the total plant biomass is the greatest irrespective of the sowing period and soil moisture. After spring sowing the contribution of aboveground organs and roots to the total biomass is greater in comparison with biomass distribution after latesummer sowing, when a greater percentage of sward and stubble was found. The tested species differ with regard to the biomass distribution. A. elatius and D. glomerata are characterized by the greatest total biomasses, while F. pratensis – the lowest. B. inermis form the greatest aboveground biomass and a great root mass, but lower in comparison to the other species biomass in the sward and stubble layer. L. perenne is characterized by great root mass and sward together with stubble, but the lowest aboveground biomass (short species).