In 1987-1989 a lysimetric experiment was designed to determine the effect of species composition of grass cultures (mono- and polyculture cultures) on Collembola communities, and to examine correlations between the abundance of Collembola and other soil organisms. Also the dependence of Collembola abundance on plant biomass was examined. Two plant communities were under study: simplified (Dactylis glomerata - monoculture) and comprising several species (a mixture of six grass species - polyculture). Plastic 0.16 m^2 lysimeters were used, filled with alluvial soil made up of coarse sand. The soil was watered and fertilized with earthworm casts. The simplification of the plant community had an unequivocal effect on the density and species diversity of collembolans. Watering and fertilization accounted for an increase in the density of collembolans only in the upper soil layer of the severalspecies plant community. Fertilization had a shortterm effect, noticeable within 30 days after the introduction of casts. Cryptopygus bipunctatus, a pioneer species, was the dominant, accounting for over 50% of the community in the upper soil layer and for 30% in the lower layer.