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 Competitiveness of Brachiaria decumbens cv. Basilisk and Stylosanthes guianensis cv. Minerão was investigated either without root restriction or by separating their root systems with a fine mesh or a solid barrier in the presence or absence of mycorrhiza (Glomus clarum). Nitrogen transfer between the legume and the grass was assessed with the ¹⁵N isotope dilution technique using a relatively stable ¹⁵N-enriched soil derived from a long-term labelling experiment. During establishment, legume development was severely restricted by competition from the grass in pots without a root barrier. However, as the system became N limited, the legume became dominant due to its access to atmospheric N₂ which contributed over 80% of the legume N requirements. S. guianensis was highly mycotrophic and inoculation with mycorrhiza favoured rapid establishment even in the treatments with no root barrier. Only …