We describe a case of peritonitis with isolation of a group JK corynebacterium from the peritoneal effluent in a patient undergoing continuous ambulatory peritoneal dialysis and treated with corticosteroids. Therapy with intraperitoneal vancomycin resulted in a rapid eradication of the organism. However, only 1 month after discontinuation of the 26-day therapy, a second episode of peritonitis with JK corynebacterium occurred. After vancomycin was restarted, the organism disappeared again from the peritoneal fluid, but the patient died a few days later from heart failure apparently unrelated to the infection. Some authors have mentioned the isolation of diphtheroids (without further identification) from peritoneal effluent of continuous ambulatory peritoneal dialysis patients, but to our knowledge, this is the first report of peritonitis associated with JK corynebacterium, an opportunistic organism that must be differentiated from other corynebacteria.