Retroperitoneal abscesses are very uncommon clinical conditions. The characteristically vague symptomatology of retroperitoneal abscess and the inherent difficulty of identifying retroperitoneal disease by physical examination contributed to these dismal therapeutic outcomes. We present an unusual case of lethal retroperitoneal abscess, caused by Citrobacter diversus(koseri), treated with surgical drainage. Citrobacter species have rarely been involved in deep tissue infection and there is no reported case of lethal retroperitoneal abscess caused by C. koseri. This case is the only reported case of C. koseri as the sole pathogen associated with a lethal retroperitoneal abscess in immunocompetent patient. The case is also notable because it confirms the recent bacterial resistance to beta-lactam antibiotics and to other antimicrobial agents, like chloramphenicol or cotrimoxazol.