Pneumatosis cystoides intestinalis (PCI) is an uncommon disease of previously unknown etiology, characterized by the presence of gas-filled cysts in the walls of some portion of the gastrointestinal tract. This investigation showed that PCI was readily produced in the germfree rat by inoculating its peritoneal cavity with a pure culture of either C. perfringens or C. tertium. Similar inoculation of the germfree animal with any one of eight other clostridia species did not result in the formation of PCI. The bacterial theory for the formation of at least some cases of PCI is now established. This work suggests that part of the treatment of those fulminating diseases known to be frequently associated with PCI should be directed at controlling a possible clostridial infection.