The roles of mycorrhiza in facilitating the acquisition and transfer of carbon (C) and nitrogen (N) to adult orchids are poorly understood. Here, we employed isotopically labelled sources of C and N to investigate these processes in the green forest orchid, Goodyera repens. Fungus-to-orchid transfers of C and N were measured using mass spectrometry after supplying extraradical mycelial systems with double-labelled [13C-15N]glycine. Orchid-to-fungus C transfer was revealed and quantified by radioisotope imaging and liquid scintillation counting of extraradical mycelium following 14CO2 fixation by shoots. Both 13C and 15N were assimilated by the fungus and transferred to the roots and shoots of the orchid. Contrary to previous reports, considerable quantities (2.6% over 72 h) of fixed C were shown to be allocated to the extraradical mycelium of the fungus. This study demonstrates, for the first time, mutualism in orchid mycorrhiza, bidirectional transfer of C between a green orchid and its fungal symbiont, and a fungus-dependent pathway for organic N acquisition by an orchid.