The purpose of the presented studies was to determine the species composition of the fungi occurring on the underground parts of potato at an thesis and to establish the qualitative and quantitative composition ofmicroorganisms living in the soil environment of this plant. Besides, the studies aimed at finding antagonistic microorganisms inhibiting potato infection by soil-borne phytopathogens. The mycological analysis showed that the symptoms of necrosis on the roots and the stem base of potato were caused by Alternaria alternata, Fusarium spp., Colletotrichum coccodes and Rhizoctonia so/ani. The microbiological analysis of the potato rhizosphere gave twice as many bacteria and fungi as from the non-rhizosphere soil. The dominating pathogenic fungi in the examined soil samples were Fusarium spp., A. alternata and R. so/ani. Three times as many antagonistic bacteria Pseudomonas spp. and Bacillus spp. and more than twice as many antagonistic fungi (Penicillium spp., Trichoderma spp., Gliocladium spp.) were obtained from the potato rhizosphere as compared to the non-rhizosphere soil.