floristic groups among arboreous communities were analyzed in different regions of pará state, using 34 different forest inventory (24 on "terra firme" and 10 on "várzea" forests) data basis. the jaccard index was used to calculate the matrix of floristic similarity, which was turned into a euclidean matrix of distance, and the ward method to define groups. through the results it was possible to conclude that the floristic composition of varzea and terra firme forests are quite different. few species occur in both ecosystems; terra firme forest shows a higher tree species richness than varzea forest. there was a cluster tendency of terra firme forests, more related to the anthropic situation and geographic proximity than varzea forests; in general, the forests formed clusters according to a decreasing order of importance, such as: soil hydric saturation, anthropic situation and geographic proximity.