Community-based studies of diarrhea etiology and epidemiology were carried out from July 1982-June 1984 in 153 infants residing in a poor peri-urban community near Lima, Peru. Study infants had nearly 10 episodes of diarrhea in their first year of life. Diarrhea episodes were associated with organisms such as Campylobacter jejuni, enterotoxigenic and enteropathogenic Escherichia coli, Shigella, rotavirus, and Cryptosporidium. These organisms appeared to be transmitted to infants in the home through animal feces, through contaminated water and food, and by direct person-to-person contact. A particularly important route of transmission may have been weaning foods, which were often contaminated because of improper preparation and inadequate cleaning of utensils. Improved feeding practices, along with avoidance of animal feces and improved personal and domestic hygiene, should be considered important interventions in reducing the high incidence of diarrhea in infants in developing countries.