Sustainable production from plantation forests depends on conserving organic matter and availability of soil nutrients, especially in short rotation stands on low fertility sites. The introduction of nitrogen-fixing trees may be an option to manage nitrogen and organic matter in those plantations where nitrogen may limit growth. This paper explores the effect of Eucalyptus grandis and Acacia mangium mixtures on tree growth, biomass allocation, net primary production, and estimation of the atmospheric nitrogen fixation. The acacia understorey in mixed-species stands did not influence biomass production and partitioning in the eucalypt overstorey, resulting in 10% higher total biomass accumulation in the mixture with A. mangium at 50% of the E. grandis density (50A:100E) than in the pure eucalypt stand (0A:100E), from age 9 months onwards. Total net primary production amounted to 31 t ha-1 and 49 t ha-1 from 18 to 30 months of age in pure acacia (100A:0E) and pure eucalypt (0A:100E) stands, respectively. Acacia net primary production in 50A:100E was only 3 t ha-1 throughout the same period due to high interspecific competition. At age 30 months, the amount of nitrogen fixed was estimated at about 30 kg N ha-1 for A. mangium planted in 50A:100E by the 15N dilution method, and about 65 kg N ha-1 for 100A:0E by the 15N natural abundance method.