Pneumatosis cystoides intestinalis (PCI) is an uncommon condition of unknown cause characterized by the presence of gas-filled cysts within the walls of some portion of the gastrointestinal tract. In this investigation, gas-filled cysts were produced in the omentum, wound, mesentery, and intestine of the germ-free rat by injecting a pure culture of Clostridium perfringens into either the wall of the distal part of the small intestine and cecum, or into the peritoneal cavity. The disorder appeared to be self-limited in time, to be related to the amount of contamination, and to occur after minimal trauma to the intestines. These experiments prove that PCI can be caused by bacteria alone. Attempts to produce the lesion by monocontaminating germ-free rats in a similar manner with pure cultures of any one of nine other common intestinal bacteria were unsuccessful.