This web page discusses Pfiesteria piscicida, an organism that has been associated with fish ulceration disease and fish kills. In its toxic state, this organism can reportedly cause dermal ulcerations to fish, possibly making them less responsive to environmental stimuli. This invites bacteria, parasites and other predatory organisms to prey on the vulnerable fish. Researchers are working to understand this organism by defining its geographic distribution and apparently complex life cycle, as well as the impact P. piscicida may have on the environment and human health. This page includes a background of P. piscicida (including life cycle, distribution, effects on fish, and environmental conditions), clinical presentation, diagnosis (exposure, signs and symptoms), management and treatment, molecular mechanism of action (identification of toxins), and references.