Editors' Summary: Pfiesteria piscicida, a sometimes toxic microorganism, is responsible for the death of millions of fish in Delaware, Maryland, North Carolina, and Virginia. Although the problem of Pfiesteria-related fish kills is associated with the Mid-Atlantic region, other toxic microorganisms have threatened fish and marine wildlife throughout the world. Scientists attribute this "global epidemic" of toxic microbes to excessive nutrient loading from nonpoint sources of pollution. Thus, toxic outbreaks of Pfiesteria appear to be a symptom of the national problem of nonpoint source pollution. This Comment discusses the probable causes and demonstrated effects of toxic forms of Pfiesteria. It summarizes existing federal and state laws that can be used to reduce nonpoint source pollution and lessen the likelihood of future outbreaks of Pfiesteria. It also examines how different entities, including federal and state governments, environmental organizations, and industry groups, have responded to toxic outbreaks of Pfiesteria. This Comment concludes that federal controls on nonpoint source runoff are needed to achieve the FWPCA's goals and to reduce the likelihood of future outbreaks of and other toxic microbes.