Spruce and birch seedlings were grown together in boxes filled with unsterile pear. Both seedlings were colonized by the ectomycorrhizal fungus Scleroderma citrinum. The two plants thus shared a common external mycelium. N-15-labelled ammonium was supplied exclusively to the fungus, while the birch or the spruce plant was continuously fed with C-13-labelled CO2 for 72 h. The carbon and nitrogen transfer rates were strikingly different for birch and spruce seedlings. The mycorrhizal mycelium received carbohydrates mainly from the birch plant and the nitrogen transfer by the fungus to the plants was largely directed towards the birch. Carbon assimilates were also transferred in both directions between birch and spruce; however, there was no conclusive evidence for a net transfer of carbon between the plants.