A pericardial effusion was diagnosed by echocardiography in a 49 year old man who suffered acute cough, orthopnea, and chest pain. Because of a positive tuberculin skin test, mycobacteria were initially suspected as the cause of the pericarditis. The patient was therefore treated with antituberculosis drugs. The pericardial effusion failed to resolve, however, and pericardiectomy was performed. Culture of the pericardial fluid yielded pure Fusobacterium nucleatum growth. The patient responded to antibiotic therapy and was in good health 3 weeks after being discharged from the hospital. This represents the first report of F. nucleatum pericarditis.