Changes in plant and macroinvertebrate communities were found following replacement of extensive zones of floating-attached/submergent native vegetation within Fitzroy River backwaters by the major environmental weed Hymenachne amplexicaulis (Poaceae). Impacts of H. amplexicaulis on native littoral flora and fauna (macroinvertebrates and fish) were assessed by comparing three sites previously supporting native vegetation and now invaded by H. amplexicaulis with nearby stands of native backwater vegetation. Plant biomass of Hymenachne plant beds was thirty-fold greater than Native plant beds whereas plant species diversity (richness) was significantly less. Macroinvertebrate communities of Hymenachne beds were significantly lower in abundance of insect orders Ephemeroptera, Hemiptera and Odonata while Coleoptera were more abundant in Hymenachne beds. Non-metric multidimensional scaling ordination of macroinvertebrate Family abundance and composition data showed that Hymenachne plant beds had a different assemblage to that in Native plant beds. In common with other studies of weed invasions, an increased abundance of some vertebrate fauna was observed (in this case an introduced fish species Xiphophorus maculatus comprised 75% of fish captured in Hymenachne beds compared with 0% in Native plant beds). Change in vegetation structure was implicated as an important factor influencing macroinvertebrate and fish faunal composition and with potential to impact on waterbird habitat values of wetlands. and decline in habitat value of Hymenachne-invaded wetlands are discussed