The aluminium detoxification mechanism of an ectomycorrhizal fungus, Suillus variegatus (Fr.) O. Kunze, grown on Petri dishes was studied using an atomic absorption spectrometer (AAS) and scanning transmission electronmicroscope with an electron dispersion photometer (STEM-EDS). Higher concentrations of soluble Al in the growth medium resulted in abundant formation of aluminium polyphosphate granules in the hyphae, which have not been noted earlier in ectomycorrhizal fungi or in their mycorrhizas. EDTA washing and STEM-EDS demonstrated that the granules were mainly located in the cell walls. The results suggest that growth of the fungal mycelium decreased as a result of higher Al and lower P concentrations inside the cells, as measured by AAS. The aluminium polyphosphate granules also contained varying amounts of other elements such as Ca, Cl, K, S, and Si.