Ectomycorrhizal fungi, including Cenococcum geophilum SIV (Cg SIV), and Pisolithus tinctorius 2144 (Pt 2144), 441 (Pt 441) and XC1 (Pt XC1), were cultured in Pachlewski liquid medium with H2KPO4,KCl- saturated vermiculite and mica as K sources, respectively, to investigate the mechanism of K absorption and mobilization by the fungi. Fungal growth rate, K absorption and mobilization varied significantly among the fungal species. Faster growth and greater K accumulation in Pt XC1 than Pt 2 144, Pt 441 and Cg siv were observed. Ectomycorrhizal fungi depressed HCl-soluble K in minerals after successive extractions by water and NH4OAc. Ratio of the total amount of K, including water-, NH4OAc- and HCI-soluble K, lost from substrates to the K accumulated in fungal colonies was less than 60%. These reveal that the ectomycorrhizal fungi could utilize K in interlayer and structural pools, which are usually unavailable for plants in short period. Large differences in the depletion of K in interlayer and structural pools by fungi were observed at fungal harvest. Taking into account the nutrient absorption by ectomycorrhizal fungi in symbionts and the direct contact between hyphae and soils, the fungi species colonized on the root surfaces seemed to be related to the effectiveness of mycorrhizas to utilize K in soils. Ectomycorrhizal fungi differed in the efflux of protons and oxalate. Pt XC1 was observed to have greatest ability to effuse protons and oxalate among the fungi adopted in the experiment. Furthermore, the higher the concentrations of protons and oxalate in the liquid culture solutions, the larger the depletion of K in interlayer and structural pools in minerals by fungi. Protons could replace interlayer K and chelation of oxalate with Fe and Al in crystal lattice could cause weathering of clay minerals. So, protons and oxalate produced by ectomycorrhizal fungi might play an important role in K mobilization in these two pools. Ectomycorrhizal fungi, including Cenococcum geophilum SIV (Cg SIV), and Pisolithus tinctorius 2144 (Pt 2144), 441 (Pt 441) and XCl (Pt XCl), were cultured in Pachlewski liquid medium with H2KPO4, KCl-saturated vermiculite and mica as K Fungal growth rate, K absorption and mobilization of prominently among the fungal species. Faster growth and greater K accumulation in Pt XC1 than Pt 2 144, Pt 441 and Cg Ectomycorrhizal fungi depressed HCl-soluble K in minerals after further extractions by water and NH4OAc. Ratio of the total amount of K, including water-, NH4OAc- and HCI-soluble K, lost from substrates to the K accumulated in fungal colonies was less than 60%. These reveal that the ectomycorrhizal fungi could utilize K in interlayer and structural pools, which are usually unavailable for plants in short period. Large differences in the depletion of K in inter taking into account the nutrient absorption by ectomycorrhizal fungi in symbionts and the direct contact between hyphae and soils, the fungi species colonized on the root surfaces seemed to be related to the effectiveness of mycorrhizas to utilize K in soils. Ectomycorrhizal fungi differed in the efflux of protons and oxalate. Pt XC1 was observed to have greatest ability to effuse protons and oxalate among the fungi adopted in the experiment. Furthermore, the higher the concentrations of protons and oxalate in the liquid. culture solutions, the larger the depletion of K in interlayer and structural pools in minerals by fungi. Protons could replace interlayer K and chelation of oxalate with Fe and Al in crystal lattice could cause weathering of clay minerals. So, protons and oxalate produced by ectomycorrhizal fungi might play an important role in K mobilization in these two pools.