the taxonomic composition, observed and estimated species richness, and patterns of community structure of arboreal spider assemblages in eleven sites surrounding the "banhado grande" wet plain in the state of rio grande do sul, brazil, are presented. these sites represent three different vegetational types: hillside (four sites), riparian (five sites) and flooded forests (two sites). the spiders were captured by beating on foliage and "aerial litter". a sample was defined as the result of beating on twenty bushes, tree branches or "aerial litter" clusters, which roughly corresponds to one-hour search effort per sample. fifty five samples (five per site) were obtained, resulting in an observed richness of 212 species present as adult or identifiable juveniles. the total richness for all samples was estimated to be between 250 (bootstrap) to 354 species (jackknife 2). confidence intervals of both sample and individual-based rarefaction curves for each vegetation type clearly indicated that flooded forest is the poorest vegetation type with respect to spider species richness, with hillside and riparian forests having a similar number of species. the percentage complementarity between the eleven sites indicated that all sites contain a distinct set of species, irrespective of their vegetation types. nevertheless, the spider assemblages in riparian and hillside forests are more similar with respect to each other than when compared to flooded forest. both cluster and nonmetric multidimensional scaling analyses showed no strong correspondence between the spider arboreal fauna and the three vegetation types. moreover, a mantel test revealed no significant association between species composition and geographic distance among sites.