Each year 12 million persons travel from an industrialized country to a developing country in the tropics or subtropics. These travelers experience a high rate of diarrhea caused by a wide variety of enteric pathogens acquired by ingestion of contaminated food or water. One or more pathogens can be found in the stool of a majority of ill individuals. Enterotoxigenic Escherichia coli generally are the most frequently identified pathogens, having been found in a median of 42% of travelers' diarrheal episodes in studies in Latin America, 36% in Africa, and 16% in Asia. Other pathogens that cause diarrhea in a smaller fraction of ill travelers include Shigella species, Salmonella species, Campylobacter jejuni, Vibrio, Aeromonas hydrophila, Entamoeba histolytica, Giardia lamblia, rotavirus, and 27-nm viruses, including Norwalk virus. Other organisms that may cause a fraction of the episodes of travelers' diarrhea include Plesiomonas shigelloides, enteroadherent E. coli, adenovirus or other viruses, and Cryptosporidium. Mixed infections of two or more of these pathogens also occur.