Ammonia assimilation has been followed in ectomycorrhizal roots of Fagus sylvatica. The absorption of ammonium ions was associated with a rapid synthesis of free amino acids in mycorrhizal tissues, glutamine being the most prominent. In the presence of [¹⁵N]ammonium, glutamate, glutamine and alanine became the most strongly labelled metabolites of ectomycor‐rhizas. Nitrogen‐15 nuclear magnetic resonance spectroscopy demonstrated that the glutamine amide‐N was the most highly enriched component of the extracts. Methionine sulphoximine and albizine, inhibitors of glutamine synthetase and glutamate synthase, almost completely blocked the incorporation of ¹⁵N label into amino acids and induced an accumulation of NH₄⁺. These observations suggest that in the ammonia‐fed beech ectomycorrhizas, ammonia assimilation occurs mainly via the glutamine synthetase/glutamate synthase pathway, and that glutamate dehydrogenase plays little, if any, part in this process. Alternative models for the nitrogen assimilation pathways in fungal and host tissues are presented.