Despite their renown, the life cycle of the true truffles belonging to the genus Tuberis not well known. The growth of the ascocarp is poorly understood. It is not known if a direct transfer of carbohydrates takes place between the host tree and the developing ascocarps through the ascogonial filament or whether ascocarps become independent from their hosts after several weeks or months and are able to use dead host tissues or soil organic matter as carbon and nitrogen sources. From a first work based on 13C and 15N natural abundance, we found that Tuber ascocarps do not exhibit a saprotrophic strategy during their development [1]. However, in situ13C and 15N labelling experiments are the only way to solve the question of carbon and nitrogen allocation during Tuber ascocarp differentiation. A first in situ13CO2pulse-labelling experiment was carried out in 2010 on a 20-year-old hazel tree mycorrhized with Tuber melanosporum. The preliminary results showed that the transfer of carbon from the leaves to the fine roots is slow but continuous during several months even during winter at low temperature. The fine roots act as a pipe to transfer the carbon to the mycorrhizas. From the mycorrhizas,13C accumulates into the ascocarps, which constitutes a carbon sink. These results contradict the statements of recognized truffle handbooks and could be of some importance for the improvement of truffle cultivation methods.