Anaerobic arthritis due to Bacteroides fragilis was diagnosed in a 33-year-old patient with prolonged fever suffering from sickle cell/thalassaemia. The causative agent was isolated from blood and purulent synovial fluid from both knee joints. A combination of chloramphenicol plus metronidazole treatment proved effective. Anaerobic arthritis has not previously been described in patients with sickle cell disease, and may reflect the well-known susceptibility of these patients to pathogens.