This International Standard specifies a method for the determination of the lethal effects of toxicants to Thamnocephalus platyurus test organisms after 24 h exposure. A second method (rapid test) is described in Annex A for the determination of sublethal effects after a very short exposure time (1 h). The methods are applicable to: a) chemical substances which are soluble or which can be maintained as stable suspensions or dispersions under the conditions of the test; b) industrial or sewage effluents, treated or untreated, if appropriate after decantation, filtration or centrifugation; c) fresh waters; d) aqueous extracts; e) toxins of blue-green algae. This International Standard is not applicable to the testing of unstable chemicals (hydrolysing, absorbing, etc.) in water unless exposure concentration is measured, nor to the testing of aquatic samples from the estuarine or marine environment.