We report a case of septic arthritis of an interphalangeal joint and osteomyelitis of the phalanx due toMycobacterium malmoense in a 61-year-old man with a 20 year history of rheumatoid arthritis treated with steroids and azathioprine. This was successfully treated with ethambutol, rifampicin and clarithromycin. To our knowledge this is the only reported case of septic arthritis due to this pathogen which is usually associated with respiratory disease or cervical lymphadenitis.