This International Standard specifies a method for the determination of the lethal effects of toxicants to Brachionus calyciflorus after 24 h exposure. The method is applicable to: a) chemical substances which are soluble, or which can be maintained as a stable suspension of dispersions under the conditions of the test; b) industrial or sewage effluents, treated or untreated, if appropriate after decantation, filtration or centrifugation; c) freshwaters; d) aqueous extracts.