A case of Listeria monocytogenes endocarditis in a patient with mitral prosthetic valve, left atrial thrombus and colonic adenocarcinoma is reported. Vegetations were not demonstrated by transesophageal echocardiography and the clinical course was benign and without complications. Cure was achieved with antibiotic therapy, and surgery was not required. These features suggest that atrial thrombus could be the source of infection.