The estuarine dinoflagellate Pfiesteria piscicida gen. et sp. nov. produces exotoxin(s) that can be absorbed from water or fine aerosols. Culture filtrate (0.22 μm porosity filters, >250 toxic flagellated cells/ml) induces formation of open ulcerative sores, hemorrhaging, and death of finfish and shellfish. Human exposure to aerosols from ichthyotoxic cultures (≥2000 cells/ml) has been associated with narcosis, respiratory distress with asthma‐like symptoms, severe stomach cramping, nausea, vomiting, and eye irritation with reddening and blurred vision (hours to days); autonomic nervous system dysfunction [localized sweating, erratic heart beat (weeks)]; central nervous system dysfunction [sudden rages and personality change (hours to days), and reversible cognitive impairment and short‐term memory loss (weeks)]; and chronic effects including asthma‐like symptoms, exercise fatigue, and sensory symptoms (tingling or numbness in lips, hands, and feet; months to years). Elevated hepatic enzyme levels and high phosphorus excretion in one human exposure suggested hepatic and renal dysfunction (weeks); easy infection and low counts of several T‐cell types may indicate immune system suppression (months to years). Pfiesteria piscicida is euryha‐line and eurythermal, and in bioassays a nontoxic flagellated stage has increased under P enrichment (≥ 100 μg SRP/L), suggesting a stimulatory role of nutrients. Pfiesteria‐like dinoflagellates have been tracked to fish kill sites in eutrophic estuaries from Delaware Bay through the Gulf Coast. Our data point to a critical need to characterize their chronic effects on human health as well as fish recruitment, disease resistance, and survival.