A case of Cardiobacterium hominis endocarditis involving a prosthetic heart valve is reported, and 12 previously reported cases are reviewed. This gram-negative rod grows slowly in blood cultures and is readily suppressed for long periods of time by token antibiotic administration. It should be considered as a causative agent in culture-negative cases of endocarditis and also when a gram-negative rod is isolated which is sensitive to all antibiotics. The treatment of choice is penicillin.