Transfer of N-15 between interacting mycelia of a wood-decomposing fungus (Hypholoma fasciculare) and an ectomycorrhizal fungus (Tomentellopsis submollis) was studied in a mature beech (Fagus sylvatica) forest. The amount of N-15 transferred from the wood decomposer to the ectomycorrhizal fungus was compared to the amount of N-15 released from the wood-decomposing mycelia into the soil solution as N-15-NH4. The study was performed in peat-filled plastic containers placed in forest soil in the field. The wood-decomposing mycelium was growing from an inoculated wood piece and the ectomycorrhizal mycelium from an introduced root from a mature tree. The containers were harvested after 41 weeks when physical contact between the two foraging mycelia was established. At harvest, N-15 content was analyzed in the peat (total N and (NH4+)-N-15) and in the mycorrhizal roots. A limited amount of N-15 was transferred to the ectomycorrhizal fungus and this transfer could be explained by (NH4+)-N-15 released from the wood-decomposing fungus without involving any antagonistic interactions between the two mycelia. Using our approach, it was possible to study nutritional interactions between basidiomycete mycelia under field conditions and this and earlier studies suggest that the outcomes of such interactions are highly species-specific and depend on environmental conditions such as resource availability.