Copper (Cu) and uranium (U) are of potential ecotoxicological concern to tropical freshwater organisms in northern Australia as a result of mining activity. No local data on the toxicity of these metals to tropical freshwater algae are currently available. The aim of this study was to investigate the effect of pH (5.7 and 6.5) on the toxicity of Cu and U to the green alga Chlorella sp. in a synthetic softwater representative of fresh surface waters in sandy-streams of tropical northern Australia. The effects of Cu and U on algal growth (cell division) rate after a 72-h exposure were determined. Intracellular and extracellular (membrane-bound) metal concentrations at the two selected pH values were also compared. Based on the 72-h minimum detectable effect concentrations (MDEC), Chlorella sp. was approximately 20-fold more sensitive to Cu (0.7 and 1.4 µg l(-1) at pH 6.5 and 5.7, respectively) than U (13 and 34 µg l(-1) at pH 6.5 and 5.7, respectively), and more sensitive than other Australian tropical freshwater organisms. The toxicity of Cu and U was highly pH-dependent. Copper concentrations required to inhibit growth (cell division) rate by 50% (72-h EC(50)) increased from 1.5 to 35 µg l(-1) as the pH decreased from 6.5 to 5.7. Similarly, the 72-h EC(50) values for U increased from 44 to 78 µg l(-1) over the same pH range. Calculation of Cu and U speciation using the geochemical model HARPHRQ, showed that differences in the concentrations of the free metal ions (Cu(2+) and UO(2)(2+)) were only minimal (<10%) between pH 5.7 and 6.5. The decreased toxicity at pH 5.7 was due to lower concentrations of cell-bound and intracellular Cu and U compared to those at pH 6.5. These results are explained in terms of the possible mechanism of competition between H(+) and the metal ion at the cell surface.