One of the main steps towards the conservation of forests is the study of composition, structure, and relationships with environmental variables. Therefore, we sampled the tree community of a fragment of semideciduous montane montana, located in Socorro, Sõo Paulo, basin of Rio Mogi Guaçu (22°32'S and 46°34'W, altitude 750 m ), in order to understand structure, species composition and spatial variation in relation to soil characteristics and edge. We analyzed the following variables: density, basal area and size distribution of trees, as well as composition, distribution and diversity of tree species. Sampling of individuals with diameter at breast height (DBH) ≥ 5 cm was done on 25 plots, 20 × 20m, where edaphic data were also collected. We recorded 1,360 individuals, 166 species, 103 genera and 41 families, as well as four soil groups: three Argisoil and one Gleisoil. Canonical correlation analysis showed that the density of the species varied in the plots and soil types, but showed no significant differences. Edge distance, aluminum, organic matter, hydrogen potential and base saturation of soils were the environmental variables most related to the floristic and structural variations of the forest. Key words: Basin of Mogi-Guaçu River, Atlantic Forest, environmental variables.