We studied Cs-137 contamination and radionuclide transfer in mountain forest ecosystems from Bulgaria. Here we show the first analyses of Cs-137 soil-to-plant transfer and we assess that it depends on the soil organic matter content and the specifics of the tree species. The forest litter is strong polluted with Cs-137 and is a barrier for its migration. In the upper 0-5 cm of the soil the Cs content ranges from 52 to 81 Bq kg–1 then decreases in deeper layers. The 1-year-old needles of spruce, fir, and Scots pine accumulate more cesium than the fine roots. The values of the needles transfer factor range between 0.3 and 0.58. The fine roots transfer factor varies between 0.1 and 0.32.