Indigenous species of Acacia are common in African savannas that have N-rich soils. This raises doubt regarding the extent of plant dependence on N2 -fixation. Why would Acacia spp. enjoy an advantage over other tree species on N-replete soils? We tested the hypothesis that competition by grass for nutrients would induce increased nodulation of Acacia karroo, A. nilotica, A. tortilis and A. nigrescens seedlings that would enable them to survive better than a non-nodulating congeneric species (A. ataxacantha). A glasshouse pot experiment was conducted to determine the capacity of the Acacia spp. to nodulate. The Acacia spp. were also grown in a randomized field plot experiment in Hluhluwe-iMfolozi Park with and without grass coexistence, and biomass accumulation and δ15N values were determined. We also sampled a range of legume and non-legume saplings from the Hluhluwe-iMfolozi Park for δ15N values. In the pot experiment all species, except A. ataxacantha, which is a forest margin species, nodulated and consequently had lower δ15N isotope values than A. ataxacantha. In the field experiment the δ15N values of the plants grown with grass were significantly lower (δ15N = 0.77 ± 0.08‰) than those grown without grass (δ15N = 5.0 ± 0.16‰) for all species, except A. ataxacantha. The δ15N isotope abundances of field-collected leaves of legume saplings were found to be significantly lower than those of non-legume species (legume δ15N = 0.98 ± 0.32‰, non-legume δ15N = 2.15 ± 0.32‰). These data confirm that A. karroo, A. nilotica, A. tortilis and A. nigrescens seedlings are capable of nodulating, and do so in their native habitat. The decreased δ15N in plants grown with grass indicates that N2 fixation was strongly enhanced by competition with grass for N. N2 fixation may thus be an important attribute allowing legume tree seedlings to survive competition with grass through a critical period when tree roots and grass roots must compete for nutrients.