In this study, the toxicity of mercuric chloride (HgCl2), an important pollutant threatening water resources for many years, and the effects of water temperature and hardness on the toxicity in cultured rainbow trout Oncorhynchus mykiss (4.79 +/- 0.16 g; 7.38 +/- 0.24 cm; mean +/- SD) were investigated at different temperatures (12 and 17 degrees C) and hardness concentrations (35, 70 and 120 mg l(-1) as calcium carbonate, CaCO3). For this purpose, the acute toxicity tests were performed by 96-h static tests in different water temperatures and water hardness concentrations. For acute toxicity tests, solutions ranging from 0.4 to 1.2 mg l(-1) were used at 12 degrees C and solutions ranging from 0.4 to 1.0 mg l(-1) at 17 degrees C. The LC50 values of HgCl2 that killed 50% of rainbow trout within 96 h in the hardness concentrations of 35, 70 and 120 mg l(-1) CaCO3 were calculated using probit analysis, and were found to be 0.725, 0.788, 0.855 mg l(-1) at 12 degrees C and 0.670, 0.741, 0.787mg l(-1) at 17 degrees C, respectively. Consequently, the toxicity of HgCl2 on rainbow trout decreased when the temperature decreased from 17 to 12 degrees C. Toxicity increased when the hardness decreased from 120 to 35 mg l(-1) CaCO3. In contrast to temperature, water hardness presents a negative effect on the toxicity of HgCl2.