Knowledge of aboveground biomass stock in Seasonal Deciduous Forests is imperative for the implementation of mechanisms to reduce emissions from deforestation, forest degradation and land reclamation. The present study analyzed the vertical distribution of aboveground biomasses in a Seasonal Deciduous Forest in Rio Grande do Sul state, Brazil. Seven 12 x 12 m plots were established, and all trees inside the plots were weighed directly in the field. Subplots of 5 x 5 m and 1 x 1 m were marked within the main plots to quantify the remaining vegetation. Average dry aboveground biomass was 316.5 Mg ha-1, trees with diameter at breast height (DBH) greater than 10 cm accounting for over 89% of this biomass. Therefore, biomass determination of large trees deserves special attention, since they represent a large part of the biomass of this forest ecosystem. Biomass of plants taller than 1.3 m and with diameter at breast height < 5 cm was 6.9 Mg ha-1, and that of plants lower than 1.3 m was 1.5 Mg ha-1. Average litter mass was 15.6 Mg ha-1. Trees of large diameters must be analyzed very carefully for quantify the biomass and carbon in the forests, because few individuals might represent a large part of the biomass of a forest ecosystem.