Question: Is soil hydrology an important niche-based driver of biodiversity in tropical forests? More speciÔ¨Åcally we asked whether seasonal dynamics in soil water regime contributed to vegetation partitioning into distinct forest types. Location: Tropical rain forest in northwestern Mato Grosso Brazil. Methods: We investigated the distribution of trees and lianas ‚â• 1 cm DBH in ten transects that crossed distinct hydrological transitions. Soil water content and depth to water table were measured regularly over a 13-month period. Results: A detrended correspondence analysis (DCA) of 20 dominant species and structural attributes in 10 vó 10 m subplots segregated three major forest types: (1) high-statured upland forest with intermediate stem density (2) medium-statured forest dominated by palms and (3) low-statured campinarana forest with high stem density. During the rainy season and transition into the dry season distinct characteristics of the soil water regime (i.e. hydro-indicators) were closely associated with each vegetation community. Stand structural attributes and hydro-indicators were statistically different among forest types. Conclusions: Some upland species appeared intolerant of anaerobic conditions as they were not present in palm and campinarana sites which experienced prolonged periods of saturation at the soil surface. A shallow impermeable layer restricted rooting depth in the campinarana community which could heighten drought stress during the dry season. The only vegetation able to persist in campinarana sites were short-stat- ured trees that appear to be well-adapted to the dual extremes of inundation and drought.