a phytosociological survey was conducted at one area belonging to the seridó ecological station to test the hypothesis that the vegetation at the seridó region is open and low. all woody plants with stem perimeter at breast height > 3 cm were counted, within 100 plots, 10×10 m, and each one had its height, stem perimeter, and crown projection diameters measured, and its aboveground biomass estimated. fifteen species were found, belonging to 15 genera and 10 families, with a shannon diversity index of 1,94 . these values are below those of most of caatinga places elsewhere. mimosoideae was the family with most species (three). all structural variables, except density, (3,250 plant ha-1, basal area 6.1 m2 ha-1, maximum height 9.5 m, maximum stem diameter 37 cm, total crown area 8,723 m2 ha-1, biomass 25 mg ha-1) had lower values than caatinga elsewhere. aspidosperma pyrifolium mart. was the dominant species comprising about half of total density, basal and crown areas and biomass. the small size of plants and the openness of the vegetation is confirmed, indicating that seridó has a physiognomic type distinct from other caatinga ones.