Electron microscopy revealed that 80% of captured vervet monkeys, held in quarantine for experimental use, showed extensive proliferation of spiral-shaped bacteria on the mucosal epithelium of the large bowel. A consortium, consisting of a predominant spirillum together with a spirochaete, was usually seen as a lawn covering the colonic epithelium. Sparsely populated areas showed preferential colonization of the tubular glands. Pathological changes were minimal, being confined to the microvillus border, and affected animals showed no evidence of distress. These findings are compared with those of a similar condition known as "intestinal spirochaetosis" reported in other primates, including man.