N2-fixation by leguminous trees can be an important source of nitrogen in forest plantations, but little is known about its seasonal variations. We estimated N2-fi xation by Acacia mangium growing in pure stands (100A) and in association with Eucalyptus grandis (50A:50E) in Brazil. At 27 months of age, we applied ammonium sulphate (99.8 atom % 15N) diluted in water (1.1 kg N/ha) to the litter. Every 3 months during one year we selected four trees per species in each treatment and collected all the leaves from two branches distributed in each third of the canopy. The average percent of N derived from fi xation (%Ndfa) was 70.3±5.2% and 48.1±10.8% in 50A:50E and in 100A, respectively. The higher %Ndfa in mixture could be due to lower soil N availability than in 100A and to strong competition of Eucalyptus over Acacia for soil N uptake. The highest and lowest values of %Ndfa were observed at the end and at the beginning of the rainy season, respectively. N2-fixation showed lower seasonal variations in 50A:50E than in 100A. In this last treatment, %Ndfa was about 30% higher at the end of the rainy season than during other periods.