The toxicity of U to Daphnia magna was determined in acute and chronic tests. The 48-hr LC50 of U (VI) in Columbia River water was 6 mg L−1. Acute toxicity diminished by a factor of 7.5 as mean water hardness and alkalinity values increased from 70 mg L−1. and 57 mg L−1. to 195 mg L−1. and 130 mg L−1. respectively. This effect was most likely the result of complexation of uranyl ion with carbonate ions. D. magna reproduction was suppressed in Columbia River water at U concentrations between 0.5 and 3.5 mg L−1. Potential hazards of U to aquatic life are discussed as they relate to mining practices.