To assess the tolerance and phytoaccumulation ability of the duckweed Lemna gibba L. to copper (Cu) and nickel (Ni), the plants were exposed to different concentrations of Cu and Ni (0.1-2.0 mg/l) under laboratory conditions. The results showed that Cu and Ni were tolerated by L. gibba at concentrations ≤0.3 and ≤0.5 mg/l, respectively. However, plant growth decreased by 50% (I(50)) when the medium contained 0.45 mg Cu/l or 0.75 mg Ni/l. The observed LCI (lowest concentration causing complete inhibition) were 0.5 and 1.0 mg/l respectively in the presence of Cu and Ni. Results from metal analysis in plant biomass revealed a high accumulation of Cu (1.5 mg g(-1) DW), a low accumulation of Ni (0.5 mg g(-1) DW) within the plants and a corresponding decrease of metals in the water. The removal percentage of Cu was about 60-80%. We conclude that the duckweed L. gibba L. showed a higher accumulation potential for Cu from polluted water than Ni after 4 days of exposure.