The present study compared the efficiency of two unicellular green algae, Chlorella vulgaris (a commercial species from Carolina Biological Supplies Company) and WW1 (an indigenous species isolated from a local sewage treatment works, tentatively identified as Chlorella miniata) in removing Ni2+ from nickel solutions with concentration ranges similar to that in electroplating effluents. The Ni2+ removal efficiency of C. vulgaris (around 33-41%) was significantly lower than that of WW1 (more than 99%) in nickel solutions from 10 to 40 mu g ml(-1). The maximum Ni2+ uptake by C. vulgaris and WW1 under the present batch experiment was 641.76 and 1367.62 mu g g(-1), respectively. According to Langmuir adsorption isotherms the nickel adsorption capacity of WW1 (2985.07 mu g g(-1)) was two times greater than that of C. vulgaris (1282.05 mu g g(-1)). These results demonstrated that WW1 was a more powerful Ni2+ biosorbent than C. vulgaris. In both species, most Ni2+ in solution was sequestered by the algal cells within the first few minutes of treatment. The cellular Ni2+ concentration increased with the concentrations of nickel in solution. After treating Ni-containing wastewater for 24 h, both species were still capable of cell division, but the growth rate was reduced in proportion to the concentrations of nickel in the wastewaters.