Environmental features associated with the distribution of grass species are poorly known in tropical savannas, particularly at smaller spatial scales. The present study aimed to determine the relative influence of 11 environmental characteristics on grass-species composition in a savanna woodland in northern Australia. Environmental characteristics relating to woody-vegetation structure and soil, plus the long-term (14-year) fire frequency, were documented along an environmental gradient and compared with grass-species composition. Differences in grass-species composition, as well as richness and evenness, were related to differences in vegetation structure and edaphic characteristics. In particular, grass-species composition was most strongly related to plant-available moisture, the density of woody plants in the midstorey (2.0-9.99m height), and canopy and litter cover. Grass-species richness and evenness were extremely low in areas where midstorey density, canopy cover and litter cover were high, and where soil moisture content in the root zone of grasses was low. Differences in fire frequency also influenced grass-species composition, with areas that had experienced lower fire frequency during the previous 14 years having lower density of the annual grass Sorghum intrans (F.Muell. ex Benth.) and the perennial grass Heteropogon triticeus (R.Br.) Stapf, and increased dominance of the perennial Eriachne triseta Nees ex Steud. The results of the present study demonstrate a complex interplay between bottom-up environmental factors and top-down processes such as fire, as determinants of grass-species composition in tropical savannas.