A pot experiment was carried out to investigate effect of ectomycorrhizal fungi on eucalyptus growth and K bio-mobilization from soils and clay minerals. In the experiment, sands mixed with soil, KCl-saturated vermiculate and mica, respectively, were used to nurse eucalyptus seedlings which were nonectomycorrhized or ectomycorrhized by an ectomycorrhizal fungus Pisolithus tinctorius strain XC1 (Pt XC1) isolated from a forest soil from Xichang, Sichuan Province, China, and a worldwide well-known ectomycorrhizal fungus Pisolithus tinctorius strain 2 144 (Pt 2 144) obtained in Australia. More depletion of HCl-soluble K by mycorrhizas from the soil and minerals than nonmycorrhizas suggested that mycorrhizas had a great ability to mobilize K present in the interlayer and feldspar. Mycorrhizal seedlings depressed greatly K digested with HF-HClO4 from substrates after consecutive extractions of soils and minerals by water, ammonium acetate and boiling HCl, while nonmycorrhizal seedlings reduced it little if any, showing that the mycorrhizal seedlings could mobilize and then utilize the structural K in mineral lattice. Ectomycorrhizal fungi played a very important role not only in promoting the growth of eucalyptus seedlings but also in mobilizing K in soils and minerals. The infection of Pt XC1 led to a better growth of eucalyptus seedlings and more K accumulation in the seedlings than that of Pt 2 144. The large differences in K accumulation by the seedlings might be due to different abilities of the two ectomycorrhizal fungi to mobilize K in interlayer and lattice pools in the clay minerals.