The majority of metal toxicity data available for freshwater organisms have been generated in laboratory water at pH > 6.5 and hardness > 50 mg/L as CaCO3. Extrapolation of these results to soft surface waters (i.e., hardness < or = 40 mg/L as CaCO3), similar to predominant conditions in the southeastern United States, may prove challenging. For example, South Carolina has surface waters that average 20 mg/L as CaCO3, and exist at extremes of 1 and 600 mg/L as CaCO3. This research characterized the acute toxicity of Cu to Daphnia magna in waters with low hardness and low pH. The 48-h total Cu median lethal concentrations were related to water hardness over a hardness range of 8 to 51 mg/L as CaCO3. Although toxicological differences existed between water hardness of 7 and 20 mg/L as CaCO3 (p = 0.0001), differences in pH (range 5.5-8.5) did not influence acute Cu toxicity. Results of these laboratory studies will provide the data needed to more accurately predict organism response to Cu in waters with low pH and low hardness.