A public aquarium with a 4-mo history of occasional fish mortalities submitted for necropsy an adult female largemouth bass (Micropterus salmoides) that died unexpectedly. Gross necropsy revealed that the pericardial cavity was markedly distended with partially coagulated blood. Examination of the heart revealed multiple nodular masses in the area of the atrium and two small perforations on the surface of one of the nodular masses. Histopathologic exam of the atrium revealed severe fibrinonecrotic endocarditis and transmural myocarditis with intralesional bacteria. A pure culture of Edwardsiella tarda was obtained from culture of posterior kidney and spleen. An area of stagnant water that may serve as the source of E. tarda was identified, and steps to rectify this problem were taken. Low-level supersaturation was also a significant stressor; the source of the supersaturation was not identified. To our knowledge, this is the first report of cardiac tamponade in a largemouth bass.