We report a peritoneal dialysis-related peritonitis infection with Rothia mucilaginosa (R. mucilaginosa), a Gram-positive germ belonging to the normal flora of the human oral cavity. Successful treatment was achieved by intraperitoneal administration of cephazolin. This case report illustrates the potential virulence of R. mucilaginosa in patients on peritoneal dialysis. We propose to routinely perform specific staining and prolonged culturing techniques for unusual germs such as R. mucilaginosa in patients with peritoneal dialysis-related peritonitis.