Microorganisms were isolated from inside the mantle of ectomycorrhizas formed by the fungus Rhizopogon luteolus Fr. and Nordh on the roots of Pinus radiata D. Don. growing in a sandy podzol. Different isolates of these microorganisms were then tested for their effect on three stages of mycorrhizal infection : growth of the fungus into the soil, growth along the root, and mycorrhiza formation. Experiments were performed in axenic and non-axenic conditions. A significant microbial population was present within the mantle of ectomycorrhizas (Gram negative bacteria being dominant) and a majority of these had a stimulating effect on mycelial growth of R. luteolus and/or mycorrhiza formation. These results suggest that some specific microbial communities have evolved to live in close association with ectomycorrhizal fungi in their symbiotic state, with mutual benefit. The potential use of such‘helper’microorganisms for improving the efficiency of ectomycorrhizal inoculation in forest nurseries is discussed.