Abstract. The mobility and bioavailability of radionuclides of accidental origin is determined by an initial ratio of their chemical fomis in fallout, kinetics of transformation processes and environmental characteristics influential in transfer and distribution of radionuclides between environmental compartments. The objective of the paper is the development and validation of dynamic process-level model FORSUN to describe radiocaesium transfer from soil to plants and fungi in forest ecosystems. The following specific aims were pursued: 1) investigation of mechanisms of radiocaesium plant uptake, in particular, analysis of a possible role of fungi mycelium as a mediator in its transfer from soil to plant; 2) qualitative and quantitative description of long-term changes of radiocaesium content in understorey compartments of forest ecosystems; 3) prediction of n7 C s activity concentrations in berries and mushrooms on the basis of a minimum of fitting parameters. A distinctive feature of model is taking into consideration vertical redistribution of radionuclide in a root layer of soil as governing factor of changes of the soil-understorey concentration ratios during first 10-30 years after the accident. The model uses parameterization of soil-plant concentration ratio through the key physico-chemical characteristics of soils determining its biological availability: a steady state portion of exchangeable caesium in soil and content of exchangeable calcium. The method for calculating radiocaesium accumulation parameters in forest fungi by soil properties is proposed based on the potassium contents in specific fungi and soil, and also steady-state portion of exchangeable caesium in soil.