We report the case of a 41-year-old woman with severe mitral regurgitation due to infective endocarditis caused by a rare zoonotic microorganism (Capnocytophaga canimorsus). She had had a rheumatic mitral endocarditis successfully treated with antibiotics when she was 13 years old. She arrived to our attention for a fever of unknown origin. She had been bitten by her dog and medicated the wound herself. About 2 weeks later she developed a fever with values up to 39.5 degrees C. Blood cultures were initially negative but in view of her particular history (dog bite), the samples were sent to a specialized center where a Capnocytophaga canimorsus (a commensal bacterium contained in the saliva of dogs and cats) infection sensitive to ceftriaxone was detected. The antibiotic therapy was consequently modified and the patient's fever resolved. At echocardiography a mild mitral stenosis with severe regurgitation (3-4+/4+) was detected. We planned surgical mitral repair but the operative findings clearly showed the need for mitral replacement and a 29 mm size bileaflet mechanical prosthesis was implanted. The postoperative course was regular and the patient was discharged on the fifth day. We highlight the importance of a careful history and correct work-up for the diagnosis and treatment of false negative blood culture endocarditis.