This part of ISO 7346 specifies a flow-through method for the determination of the acute lethal toxicity of substances soluble in water under specified conditions to a species of freshwater fish (Brachydanio rerio Hamilton-Buchanan - common name, zebra fish) in water of specified quality. The method is applicable for assigning, for each test substance, broad categories of acute lethal toxicity to Brachydanio rerio under the test conditions. The results are insufficient by themselves to define water quality standards for environmental protection. The method is also applicable when using certain other species of freshwater fish as the test organism. The method may be adapted for use with other freshwater fish and marine and brackish water fish with appropriate modification of the test conditions, particularly with respect to the quantity and quality of the dilution water and the temperature.