the rivers and floodplain lakes of the urucu petroleum-bearing region in central amazonia are extensively colonized by aquatic macrophytes, which may be affected by accidental spills during oil extraction and transportation operations. among the species of macrophytes, the free-floating eichhornia crassipes (water hyacinth) is abundant; aim: the objective of this study was to evaluate the effect of different dosages of urucu crude oil (0, 0.5, 1.5 and 3.0 l.m-2) on the biomass of e. crassipes and on some physical and chemical characteristics of the water in which the plant grew; methods: the experiment was conducted over a period of 84 days. every seven days, the biomass (live and dead) of e. crassipes and the temperature, ph, electrical conductivity and dissolved oxygen in the water were measured; results: the dosage of 0.5 l.m-2 was sufficient to cause partial mortality (48%) in e. crassipes after 35 days of exposure to oil. the dosage of 3.0 l.m-2 caused total mortality (100%) in e. crassipes after 84 days of exposure. the decomposition of the oil and the dead biomass of e. crassipes reduced the dissolved oxygen and ph, and increased electrical conductivity and concentrations of total phosphorus in the water; conclusions: we conclude that an oil spill can cause total mortality in the local population of one macrophyte species, but not of another. this may alter the macrophyte species diversity in the impacted region. in the case of e. crassipes and pistia stratiotes, an urucu crude oil spill could favor e. crassipes, the species less sensitive to oil.