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There are more than 160 species of naturalised exotic grasses in northern Australia and these have invaded a range of freshwater and terrestrial environments. We have been studying the impacts of two exotic grasses, gamba grass (Andropogon gayanus Kunth.) and para grass (Brachiaria mutica Forssk.), which occur in tropical savanna and floodplains, respectively. Despite the very different habitats they occupy, there are striking similarities in their impacts on native vegetation and invertebrates. In both habitats, exotic grass invasion results in a shift in plant composition and a reduction of native plant species richness. These changes in plant communities have the potential to undermine biodiversity conservation goals; however, our findings suggest that not all biota are affected in the same way. Although there was a clear change in the composition of wetland and terrestrial flora (diversity and biomass) following weed invasion, freshwater macroinvertebrate communities and terrestrial ant communities showed little or no response. This is particularly interesting given that both these invertebrate groups are considered to be sensitive indicators of environmental change. Although these exotic plants almost completely displace the native vegetation in both environments, in some respects it is simply one grass being replaced by another and the change in habitat structure appears to be insufficient to affect the invertebrate communities. We suggest that, although weeds may pose a clear threat to plant biodiversity, the response of some faunal groups will be determined largely by the degree of structural change.