During July 1998, at the USGS Leetown Science Center, Kearneysville, West Virginia, an epizootic occurred in 16-month-old Atlantic salmon (Salmo salar). The cause ofmortality was diagnosed as furunculosis, a serious disease in salmonid fishes caused by the bacterium Aeromonas salmonicida. The fish were being maintained as part ofongoing research and were held in uncovered 30-m-Iong concrete raceways, each supplied with about 757 L per minute of 12 °C pathogen-free spring water. The means by which the fish became infected could not be determined, and there was no recent history offurunculosis in the hatchery system.