To determine the effects of water hardness on the toxicities of cobalt (Co) and nickel (Ni) to a freshwater fish, Capoeta fusca. Toxicity was investigated by static bioassay. Fish were exposed to cobalt (as CoCl(2)) and nickel (as NiCl(2)) for 96 h in waters with two levels of hardness ("hard" and "very hard", nominally 130 mg/L and 350 mg/L as CaCO(3), respectively). Water hardness had a significant effect on the acute toxicity of both elements. The 96 h LC(50) values for Co were 91.7 mg/L and 204.8 mg/L in hard and very hard waters, respectively, and for Ni the 96 h LC(50) values were 78.0 mg/L and 127.2 mg/L, respectively. The fish were more sensitive to Co and Ni toxicity in hard water than in very hard water; very hard water protects C. fusca against the toxicity of Co and Ni.