1.1 ,This guide describes procedures for obtaining laboratory data concerning the acute toxicity of chemicals and aqueous effluents released into fresh, estuarine, or marine waters. Acute toxicity is measured by exposing Brachionus newly hatched from cysts to a series of toxicant concentrations under controlled conditions. This guide describes a test for using B. calyciflorus, a fresh water rotifer, and the Appendix describes modifications of this test for estuarine and marine waters using B. plicatilis. These procedures lead to an estimation of acute toxicity, including the concentration expected to kill 50 ,% of the test rotifers (LC50) in 24 h. Procedures not specifically stated in this guide should be conducted in accordance with Guide E729 and Guide E1192.