Between October 1970 and June 1971, at the National Center for Primate Biology, Yersinia pseudotuberculosis of serotypes I-B and III was isolated from 9 monkeys (one during life and 8 at necropsy) of the following species: Macaca cynomolgus, Macaca nemestrina, Macaca radiata and Cercocebus fulliginosus. All these animals had characteristic gastrointestinal lesions consisting of superficial erosions or ulcerations with masses of gram-negative coccobacilli and an acute inflammatory exudate. Involvement of mesenteric nodes, livers and spleens by similar lesions was common. A more granulomatous reaction was rarely seen. Similar lesions without bacteriologic confirmation were found at necropsy in 20 other animals. When guinea pigs were inoculated intraperitoneally with our isolates, they developed focal splenic and hepatic necrosis resembling the septicemic form of the disease which is seen rarely in man. When inoculated intraperitoneally, they developed mesenteric lymphadenitis resembling human nonspecific mesenteric lymphadenitis; no intestinal lesions could be detected in the animals inoculated orally. The granulomatous component of the inflammatory response was better developed in guinea pigs than in the monkeys. It is concluded that infection by Yersinia pseudotuberculosis in nonhuman primates and probably also in other species, including man, is primarily a gastrointestinal disease. The primary intestinal lesions may be conspicuous, as in the monkeys, or inconspicuous, as in the guinea pig and man. The acuteness of the inflammatory response in the monkeys, when compared to the more granulomatous reaction in guinea pigs, suggests that the great majority of the monkeys died from an overwhelming infection before they could develop hypersensitivity to the organism.