In truffle beds planted in environments not suitable for the truffle species chosen, mychorrhizal substitution usually takes place after a period of time. The present work was aimed at verifying the capacity of mycorrhization of Tuber borchii in the presence of other Tuber species in the soil. Some young plants of Quercus pubescens were inoculated in the same conditions using two different substrates (natural and artificial substrate) and different species of Tuber as inoculum. In fall 2004, spring 2005 and in spring 2006, the percentage of mycorrhization for each of the species of fungi involved was assessed in 32% of the plants in each inoculation test. The most abundant mycorrhizae of T. borchii were detected in roots of plants grown in natural soil. But T. brumale shows remarkable percentages of mycorrhization (up to 50%) and seems to substitute T. borchii in some cases. The results indicate that truffle beds of T. melanosporum and T. aestivum planted in soils containing propagation elements of T. borchii do not give the desired production.