A clinical field trial was conducted to evaluate the efficacy of aqueous chloramine-T to control mortality of goldfish (Carassius auratus) fingerlings infected with Flavobacterium columnare. In addition, the acute toxicity of chloramine-T to goldfish was determined in glass aquaria under static conditions. Chloramine-T concentrations in the toxicity test ranged from 5.0 to 40 mg/l. The concentration of chloramine-T that killed 50% of the goldfish within 24 It (24-h LC50) was estimated as 24.3 +/- 0.6 mg/l (95% confidence limits). Two days after exposing goldfish to E columnare, fish were treated with chloramine-T ranging from 0 to 25 mg/l. None of the uninfected control fish died, and all deaths occurred between 3 and 11 days after exposure to F columnare. Mortality of the untreated control (0 mg/l chloramine-T) group was 70% while survival of fish was significantly increased with increasing chloramine-T concentration up to 15 mg/l chloramine-T. When the concentration of chloramine-T was increased from 15 to 20 or 25 mg/l, survival of fish decreased. F columnare was isolated from skin and gills of all fish that died during the experiments but was not isolated from survivors 21 days after exposure to bacteria. Results indicate that 15 mg/l chloramine-T can be used to treat columnaris disease in C auratus under the experimental conditions of this study. (C) 2004 Elsevier B.V. All rights reserved.