Diarrheal disease was studied prospectively in 35 Peace Corps volunteers during their first five weeks in rural Thailand. Twenty (57%) developed the syndrome of travelers' diarrhea. Recognized bacterial enteric pathogens were isolated from stools during 47% of 39 episodes of travelers' diarrhea. Enterotoxigenic Escherichia coli was isolated during 26% and Shigella during 13% of the episodes. Of the 20 volunteers, 50% had bacteriologic and/or serologic evidence of infection with enterotoxigenic E. coli. Sixty-one percent of isolates of enterotoxigenic E. coli and 92% of isolates of Shigella were resistant to doxycycline. Other enteric pathogens, including Campylobacter jejuni/coli, Yersinia enterocolitica, Salmonella, rotavirus, Norwalk agent, and Entamoeba histolytica, were associated with episodes of travelers' diarrhea. Aeromonas hydrophila, isolated from 31% of 39 episodes of travelers' diarrhea, was of unknown pathogenic importance. Thus, episodes of travelers' diarrhea in Thailand were associated with a variety of organisms, among which antibiotic-resistant bacterial enteropathogens were common.