A 37-year-old injecting drug user presented with signs and symptoms of septic arthritis. Staphylococcus aureus was grown from his blood cultures. Despite treatment with flucloxacillin and fusidic acid his condition continued to deteriorate. Echocardiography showed no signs of endocarditis. Culture of his knee aspirate grew Anaerococcus prevotii after 5 days of incubation. Metronidazole was added to his treatment regime. A collection of pus (800 ml) was drained from the right thigh and A prevotii was isolated. His condition improved gradually and he was subsequently transferred to a drug rehabilitation unit after completing his antibiotic course. The importance of anaerobic streptococci in septic arthritis, as a very rare cause, is highlighted.