ABSTRACTThe impact of inoculation with Paxillus involutus on the utilization of organic carbon compounds by birch roots was studied by feeding [14C]Glu or [14C]malate to the partners of the symbiosis, separately or in association, and by monitoring the subsequent distribution of 14C. Inoculation increased [14C]Glu and [14C]malate absorption capacities by up to eight and 17 times, respectively. Six‐ and 15‐d‐old mycorrhizal roots showed about four‐fold higher [14C]Glu and [14C]malate absorption capacities compared with 60‐d‐old mycorrhizal roots, suggesting that the early stages of mycorrhiza formation induced higher requirements for C skeletons. Moreover, the results demonstrated that inoculation strongly modified the fate of [14C]Glu and [14C]malate. It was demonstrated that exogenously supplied Glu and malate might serve as C skeletons for amino acid synthesis in mycorrhizal birch roots and in the free‐living fungus. Gln was the major 14C‐sink in mycorrhizal roots and in the free‐living P. involutus. In contrast, citrulline and insoluble compounds were the major 14C sinks in non‐mycorrhizal roots, whatever the 14C source. It was concluded that mycorrhiza formation leads to a profound alteration of the metabolic fate of exogenously supplied C compounds. The ecological significance of amino acid and organic acid utilization by mycorrhizal plants is further discussed.