This paper is devoted to the problem of Cs-137.. accumulation by plants of forest ecosystems in the center of the East European Plain. It has been found that shoots and roots' phytomass has the maximum difference in the accumulation of these elements depending on the K-selective transport systems of plants. The increased accumulation of Cs-137 in green moss and aboveground organs of Pteridium aquilinum bracken fern, as well as in the leaves and thin branches of oak, aspen, and birch and in the shoots of cranberries has been revealed. Analysis of the physiological aspects of nitrogen and potassium nutrition allows supposing possible explanation of the mechanism of Cs-137 hyperaccumulation by aboveground phytomass of some species.