Norway spruce (Picea abies (L.) Karst.) seedlings growing in a growth pouch system were used to investigate the effects of the ectomycorrhizal fungus Hebeloma crustuliniforme (Bull. ex St. Amans) Quél. and various Cs/K ratios on the uptake of ¹³⁴Cs, expressed as a percentage of the total amount of ¹³⁴Cs supplied. The amount of ¹³⁴Cs taken up by seedlings increased with increasing Cs/K ratio. At a Cs/K ratio of 0.1, uptake of ¹³⁴Cs ranged between 7.2 and 7.3% and was independent of ectomycorrhizal status, whereas at Cs/K ratios ≥ 1 uptake of ¹³⁴Cs varied from 8.1 to 11.1% for ectomycorrhizal and from 10.4 to 14.4% for non-inoculated plants. Ectomycorrhizal seedlings contained a lower concentration of ¹³⁴Cs than non-inoculated seedlings. Among plant parts, the amount of ¹³⁴Cs was significantly lower in needles and lateral roots of ectomycorrhizal seedlings compared with non-inoculated seedlings. Among fungal and seedling tissues, highest X-ray net counts of ¹³³Cs were measured in fungal hyphae of ectomycorrhizal mantles. X-Ray net counts of ¹³³Cs in lateral roots of ectomycorrhizal and non-inoculated plants were similar, but 5 to 10 times higher than in main roots and needles, suggesting an accumulation of ¹³³Cs in lateral roots and slow translocation to other plant parts. In contrast, X-ray net counts of K indicated that K was readily mobilized from lateral roots to main roots and needles. Elemental mapping showed a relatively homogeneous distribution of ¹³³Cs within the root.