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To determine the exchange of nitrogen and carbon between ectomycorrhiza and host plant, young beech (Fagus sylvatica) trees from natural regeneration in intact soil cores were labelled for one growing season in a greenhouse with 13CO2 and 15NO3 15NH4. The specific enrichments of 15N and 13C were higher in ectomycorrhizas (EMs) than in any other tissue. The enrichments of 13C and 15N were also higher in the fine-root segments directly connected with the EM (mainly second-order roots) than that in bulk fine or coarse roots. A strict, positive correlation was found between the specific 15N enrichment in EM and the attached second-order roots. This finding indicates that strong N accumulators provide more N to their host than low N accumulators. A significant correlation was also found for the specific 13C enrichment in EM and the attached second-order roots. However, the specific enrichments for 15N and 13C in EM were unrelated showing that under long-term conditions, C and N exchange between host and EMs are uncoupled. These findings suggest that EM-mediated N flux to the plant is not the main control on carbon flux to the fungus, probably because EMs provide many different services to their hosts in addition to N provision in their natural assemblages.