Serratia appears as a pathogen of increasing frequency and clinical significance in bone and joint infections in heroin users. This is the fifth case report of septic arthritis due to Serratia marcescens in intravenous heroin users. The clinical and laboratory features were not different from other acute pyogenic arthritides. Signs of infection were obscure even in the presence of debilitating disease. Although Pseudomonas and Staphylococcus are more common organism in bone and joint infections of heroin users, Serratia should be considered as a possible pathogen in such patients. In the present case, immediate open drainage followed by systemic Gentamicin treatment gave rapid relief of pain and restoration of full range of motion of the joint.