A 56-year-old woman with systemic lupus erythematosus developed septic arthritis and bursitis of the left shoulder due to an atypical mycobacterium, M. xenopi. Plain radiography, ultrasound (US), and MRI were performed. Articular disease by tuberculous and nontuberculous mycobacteria have similar presentations, clinically as well as radiologically, and have to be differentiated from other chronic bacterial or fungal infections, pigmented villonodular synovitis, rheumatoid arthritis, gout, hemophilia, and synovial chondromatosis. Although atypical mycobacterial involvement of the skeleton and soft tissues is relatively uncommon, its incidence is increasing, as is the incidence of extrapulmonary tuberculosis in western countries. The triad of Phemister is reemphasized, and the US and MRI findings are demonstrated. The definitive diagnosis has to be made by culturing biopsied synovium or synovial fluid.