Ozone can be used as an alternative to chlorination to control biofouling in cooling water systems. The possible negative environmental impact of a discharge of ozone-containing cooling water was investigated. The acute toxicity of dissolved ozone was determined for fish larvae of three species [Cyprinus carpio (at 27 degrees C), Leuciscus idus (at 27 degrees C) and Clarias gariepinus (at 32 degrees C)] and to Daphnia magna (at 21 and 27 degrees C). The results indicate that ozone is very harmful to aquatic life. Daphnids are more sensitive to ozone than fish larvae. The mean 48-h LC50 value for the larvae amounts to about 35 microg/liter, while the 48-h NOEC for D. magna was 11 microg/liter (at 21 degrees C). It was concluded that, to protect aquatic life, discharged cooling water should not contain any dissolved ozone. This can be achieved in practice by mixing the treated cooling water with a source of organic substances before discharge, as free ozone will react immediately with organic matter and thus disappear.