Microorganisms play an important role in the fixation of radionuclides in forest soils. In particular, fungi have the capacity to absorb and translocate radionuclides. The role of the ectomycorrhizal fungus Hebeloma crustuliniforme in the uptake of radiocaesium (¹³⁴Cs) and radiostrontium (⁸⁵Sr) into seedlings of Norway spruce (Picea abies) was investigated in a pouch test system. Inoculated and non-inoculated seedlings; seedlings inoculated during 8 and 15 weeks; seedlings exposed during 2 and 3 weeks to the radioactive solution; and seedlings grown under low and high ammonium conditions prior to the application of the radionuclides were compared. The final ¹³⁴Cs and ⁸⁵Sr activity was determined in fine-roots, main-roots, stems and needles. The results showed that ectomycorrhizae reduced the uptake of ¹³⁴Cs and ⁸⁵Sr. The degree of ectomycorrhization was of crucial importance and seemed to be governed by the period during which ectomycorrhizae were allowed to develop and by the ammonium concentration in the nutrient solution. The radionuclide uptake increased with increasing exposure time. Both radionuclides were predominantly accumulated in fine-roots. However, needles proved to describe best the result of net root uptake and translocation to the shoot. The uptake-and and translocation-rates of ⁸⁵Sr were smaller than those of ¹³⁴Cs. It is assumed that the translocation is coupled with the intensity of water fluxes through the xylem and that ⁸⁵Sr is more readily adsorbed into mycelium or plant tissue relative to ¹³⁴Cs. The effect of high ammonium growth conditions was overcome by the effect of ectomycorrhization, except in needles with a very large biomass which behaved as a strong sink and led to a high accumulation of ¹³⁴Cs.