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The dynamics of the arboreal structure of a Mixed Rain Forest was assessed in the period 1992-2003. The area belongs to the “Florestal Gateados company”, located in Campo Belo do Su county, in Santa Catarina State, Southern Brazil. The surwey was carried out in 16 sample units of 10 x 60 m (600 m²), where all individuals with DAP > 10 cm Were registered, measured and identified. After the analysis, it could be concluded that: Lithraea brasiliensis was the most representative specie in the structure of the forest in both analised periods, and this was related to the high values of density, dominance, frequency, besides the distribution in all forest layers. The most important species, which increased their participation in the composition and structure were, Ocotea pulchella, Matayba elaeagnoides, Clethra scabra, Clethra uleana, Sebastiana commersoniana and Araucaria angustifolia; The species which decreased in their importância in the forest structure were, Myrsine coriacea, Nectandra grandiflora Capsicodendron dinissi, Ilex theezans,Ilex dumosa and Xylosma ciliatifolium. The distribuition of the trees in the forest layers changed from increasing distribution in 1992 to uniformity in 2003, enhancing the increases in density of trees of the lower layer; the forest is in a successional process, indicated particularly by the dinamycs of the structure of the lower forest layers.