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The aim of this study was to evaluate fine root biomass density (FRBD) in mixed and monospecific stands of Eucalyptus grandis x E. urophylla and Acacia mearnsii (black wattle) in Bagé-RS (Southern Brazil). An experimental trial was installed with three treatments: 100% Eucalyptus (100E); 100% Acacia mearnsii (100A); 50% Eucalyptus + 50% Acacia mearnsii (50E:50A). The trial was carried using a randomized block design with three replicates. The fine root (≤ 2.0mm) biomass density was determined 8 and 18 months after planting the trees. Soil samples were collected, with a cylindrical extractor auger (d = 7.0 cm), from four depths (0 - 5, 5 - 10, 10 - 20 and 20 - 30 cm) at each sampling point. After 8 months, the FRBD distribution was the same in both species and in all soil layers, reaching the maximum projection at 125 cm from the tree trunk. After 18 months, the root biomass density was higher in the monospecific black wattle stand than in the monospecific eucalyptus stand and the mixed stand. The fine root biomass density was highest in the 5 - 10 cm layer close to the trunk, for the planting row spacing, the planting line and the diagonals between two planting lines. Knowledge about fine root growth and distribution in soil at initial stages of stand development may help in decisionmaking for intensive forestry, thus ensuring more efficient use of soil resources.