Enterotoxigenic strains of Escherichia coli were isolated from eight (16%) of 50 Mexican children admitted to the hospital with diarrhea and from one of 50 children hospitalized for nonenteric disorders. All of the toxigenic strains tested elaborated a heat-labile enterotoxin, and in seven of nine patients no E. coli capable of concomitant production of heat-stable enterotoxin were found. None of the strains of E. coli with classical enteropathogenic serotypes isolated from nine patients with diarrhea produced either heat-labile or heat-stable enterotoxin. Although the results of this study strongly suggest that enterotoxigenic strains of E. coli are probably responsible for a significant number of cases of diarrhea in an indigenous Mexican pediatric population, further proof will require demonstration of in vivo production of enterotoxin and/or antitoxin.