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The floristic diversity and similarity of the arboreal community under an Araucaria angustifolia stand (area A) and a Mixed Rain Forest (area B) located in the National Forest in Irati, Paraná State – Brazil, were compared. All the individuals with diameter (dbh) above 10 cm, were identified, numbered and positioned spatially in a cartesian system (X, Y). The floristic was evaluated in both areas and compared with the richness (Odum), diversity (Simpson and Shannon), evenness (Pielou) and similarity (Jaccard) indices. The Shannon’s index was used in a random complete block design to compare the diversity among the 35 studied blocks with 1 ha each one (100 m x 100 m). Finally, the Jaccard’s coefficient was used in the clustering analysis (Ward’s method) to verify the similarity among the blocks. The area B presented 29 species, 13 genera and 9 families more than area A. Ilex paraguariensis, Ocotea odorifera and Araucaria angustifolia are the species of larger occurrence in the area B, while in the area A presented Myrsine umbellata, Psychotria vellosiana and Casearia sylvestris. The Shannon’s indices calculated for the 10 blocks in the Araucaria stand (A) were statistically equals, and for the 25 blocks in the forest (B), the differences was significant, tested by analysis of variance. The cluster analysis for the 35 studied blocks formed 4 similar groups according to Jaccard’s coefficient. The results indicated that the arboreal vegetation in the stand is in a succession stage that comes after the initial and have limitations for its development due to the covering of the Araucaria trees.