The association of Acacia could increase Eucalyptus plantations productivity through a positive balance between facilitative effects and competition between species. In Brazil, the development of mono-specific stands of Acacia mangium (100A) and Eucalyptus sp. (100E) was compared with nitrogen (N) fertilisation treatment (100E + N) and mixed-species plantations in a 1:1 ratio (50A:50E). The study was conducted in Itatinga-SP, Sinop-MT and Colinas-TO with mean annual temperature of 19.4, 25.0 and 27.5 °C, mean annual rainfall of 1320, 2640, and 1850 mm, and dry season duration of 3, 5 and 7 months, respectively. The soils are sandy to sandy-clay. At 36 months, Eucalyptus height in 100E was 18.9, 10.9 and 13.7 m, in SP, MT and TO, respectively. For Acacia the corresponding values in 100A were 14.2, 13.5 and 10.8 m, respectively. This pattern, also observed for diameter at breast height, showed that Eucalyptus was proportionally more adapted to the ecological conditions than Acacia in SP, the opposite being found in MT. Stand basal area (SBA) was 2, 10 and 3 % higher in 100E+N than in 100E in SP, MT and TO, respectively. SBA was 7 and 1 % higher in 100E (8.33 and 3.70 m2 ha-1) than in 100A (7.77 and 3.66 m2 ha-1) in SP and TO, respectively. By contrast, SBA was 69 % higher in 100A (7.54 m2 ha-1) than in 100E (4.47 m2 ha-1) in MT. SBA was 6 and 4 % higher in 100E than in 50A:50E in SP and TO, respectively. By contrast, SBA was 21 % higher in 50A:50E than in 100E in SP. The occurrence of higher stand production in mixed species plantations of Eucalyptus and A. mangium than Eucalyptus monocultures depends on ecological conditions. When conditions permit high eucalypt stand yield as observed in SP, the potential N facilitation by Acacia cannot balance the lower potential of growth of Acacia trees, which are also deeply competed by Eucalyptus trees. By contrast, as observed in MT mixed plantations are likely more productive than Eucalyptus monoculture when the environmental