Cellulomonas spp. are often believed to be of low virulence. There are only a few reports of human infections. We report the first case of endocarditis caused by Cellulomonas in an intravenous drug abuser. The diagnosis of infective endocarditis (IE) in this case was definite using the Duke criteria. The course of the disease was complicated with a heart failure and possible mycotic aneurysm in the left leg. After the end of antimicrobial therapy aortic valve replacement was done because of severe heart failure.