A 65-year-old woman on continuous ambulatory peritoneal dialysis (CAPD) developed blood-tinged dialysate and bacterial peritonitis following a colonoscopic polypectomy. She grew multiple anaerobic organisms in her dialysate despite antibiotic prophylaxis with vancomycin and gentamicin prior to the procedure. This case confirms the need for broad spectrum antibiotic prophylaxis prior to colonoscopic procedures, especially if polypectomy is planned. The antibiotic chosen should cover anaerobes as well as gram-positive and gram-negative enteric organisms. We suggest the use of ampicillin, clindamycin, and an aminoglycoside antibiotic for this prophylaxis.