In a random household survey conducted on St. Thomas, U.S. Virgin Islands, the incidence of ciguatera fish poisoning was found to be 36.5 cases per 1,000 population per 5 years (95% confidence limits ± 16.9 cases per 1,000 population per 5 years). An average of 3.6 cases per 1,000 population per year were diagnosed in the hospital emergency room on St. Thomas. Cases seen in the emergency room occurred most frequently among persons aged 30–39 years. No clear seasonality for cases could be demonstrated. In an investigation of cases occurring between 1 January and 10 April 1980, illness was caused by a variety of different fish, with carrang (Caranx ruber) the species most commonly implicated. Patients and age-matched controls ate fish with equal frequency; patients were significantly more likely to have had previous episodes of ciguatera fish poisoning than were controls.