Synthetic detergents are reported to be acutely toxic to fish in concentrations between 0.4 and 40 mg/1. Factors affecting toxicity include the molecular structure of the detergent, water hardness, temperature and dissolved oxygen concentration; the age and species of the test fish, and acclimation to low concentrations of detergent. Some of these factors appear to be of only limited importance. Gill damage is the most obvious acute toxic effect; the immediate cause of death may be asphyxiation, but detergents may also be toxic internally. Lethal effects not related to gill damage have not been investigated. Sublethal effects include retardation of growth, alteration of feeding behaviour and inhibition of chemoreceptor organs. Low levels of detergents may also increase the uptake of other pollutants. Invertebrates, especially in their juvenile stages, are extremely sensitive to detergents: concentrations below 0.1 mg/1 interfere with growth and development in some species. The interactions between detergents and proteins, and their influence on membrane permeability may be the basis of the biological action of detergents. Detergents in natural waters are usually partially degraded, and a maximum permissible concentration of 0.5 mg/1 would probably be harmless under most conditions.