Scientists from Europe, North America and South America convened in Capri, Italy, April 24-28, 2006 for the Ninth International Symposium on Fish Physiology, Toxicology, and Water Quality. The subject of the meeting was Eutrophication: The toxic effects of ammonia, nitrite and the detrimental effects of hypoxia on fish. These proceedings include 22 papers presented over a 3-day period and discuss eutrophication, ammonia and nitrite toxicity and the effects of hypoxia on fish with the aim of understanding the effects of eutrophication on fish. The ever increasing human population and the animals raised for human consumption discharge their sewage into rivers and coastal waters worldwide. This is resulting in eutrophication of rivers and coastal waters everywhere. Eutrophication is associated with elevated ammonia and nitrite levels, both of which are toxic, and the water often becomes hypoxic. Aquatic hypoxia has been shown to reduce species diversity and reduce total biomass.