Algal assays were used to establish toxicity guidelines for natural waters. ‘Cu equivalent’ rather than the conventional EC 50 value was used as the toxicity unit. By correlating Cu dosages with%of control growth, Cu reference curves were constructed for our test species,ChlorellaandScenedesmus. These Cu curves, which compensated for species sensitivity differences, were then used to translate algal growth response to toxicity of the test waters. Expressed in Cu equivalent, our test waters displayed a toxicity range from 25 to 300 ug Cu.l‐1. After further tests with Cu on organisms such as algae,Daphniaand rainbow trout, ‘sensitive’ water where delicate plankton species could not survive, was found at Cu eq. concentrations between 90 and 120 ug.l‐1.