The total biomass of a Eucalyptus nitens plantation was estimated in age sequence from 4 to 7 years, with fertilization (X Region, 41º01’S-73º27’W), in red clayey soil of Crucero series. The forest biomass at ages 4, 5 and 7 was estimated from the corresponding average quadratic diameter (DMC) of the tree; being interpolated at age 6. Components of the crown, stem and roots were obtained; additionally humus and litter were calculated. The total biomass of the stand reached 92 t ha–1 at 4 years, 131 t ha–1 at 5 years and 181 t ha–1 at 6 years with a density of 1,500 trees ha–1. After thinning, between ages 6-7, the total biomass increase was 29.8 t ha–1. Yearly root increase was 0.5, 2.4 and 3.8 t ha–1 in the sequence 4-7 years old. Thinning provided the soil with 21.7 t ha–1 of waste (crowns and bark) and exported 49 t ha–1 of wood. Pruning of the lowest third of crowns (800 trees ha–1) at 5 years contributes with 5.8 t ha–1 of branches and 3.8 t ha–1 of leaves to the soil.