GB/T 13266-1991 is a water quality testing standard used to determine the acute toxicity of substances to fleas (macrofleas). Macrofleas are sensitive to changes in water quality and are often used as an indicator organism for toxicity testing. The standard specifies experimental conditions, such as flea foster, preparation of tested substances, detailing exposure experimental operations, and evaluating toxicity by observing indicators such as flea mortality over a certain period of time. It covers result judgment and data processing. This method provides a scientific means for assessing the toxicity of pollutants in water and ensuring aquatic ecological safety. It helps water quality monitoring and chemical environment risk control, and is of great significance to maintaining aquatic ecological equilibrate.