The epidemiology of diarrhea in colony-born M. nemestrina was studied in 205 neonates and infants in an Infant Primate Research Laboratory (IPRL), and in 248 neonates, juveniles and adolescents up to 4 years of age at a separate breeding and holding facility (Primate Field Station, PFS). Computerized medical records of individual animals over a 5-year period were analyzed to determine the incidence of diarrhea; age, duration and number of episodes; mortality and etiology. The incidence of diarrhea at the IPRL was highest in infants at less than 1 month of age (18.6 cases per 1000 animal days) and at 1-6 months olds (2.0 cases per 1000 animals days). Many infants had multiple episodes. All episodes were less than 10 days in duration. Mortality was low. At the PFS, the highest incidence occurred in infants at 6-12 months of age (1.36 cases per 1000 animal days). Multiple episodes were less common. Duration was variable. The infectious agents diagnosed at both facilities were Shigella, Campylobacter and Cryptosporidium. No pathogens were identified in many episodes. Shigella was more common at PFS than at the IPRL. Chronic diarrhea occurred in approximately 10% of animals at PFS. Intestinal amyloidosis and retroperitoneal fibromatosis were found in 13 animals with chronic diarrhea. Further studies are needed to determine the pathogenesis of chronic diarrhea, the etiologic significance of Campylobacter, and the causes of diarrhea when no pathogens are isolated.