FOR several years past, migratory fish passing through the Tees estuary have been killed in large numbers. This is most noticeable in the months of April, May, and June, during the seaward migration of young salmon (Salmo solar L.) and sea trout (Salmo truttaL.), when many thousands are found dead oil the foreshore. An extensive biological, chemical, and hydrographical survey of the river, undertaken by the Department of Scientific and Industrial Research as part of the programme of the Water Pollution Research Board, has provided an opportunity for an investigation into the cause of this mortality.