Septic arthritis due to Serratia species was seen in four users of intravenous heroin. In all cases, the organism was cultured from joint aspirates. Both the clinical presentation and the involvement of the sacro-iliac and knee joints were notably similar to the Pseudomonas septic arthritis encountered in other heroin users. All four patients responded satisfactorily to therapy although one had a residual flexion contracture of the knee. Twelve previously reported cases of Serratia arthritis are reviewed.