A woman returning from Mexico was hospitalized as an emergency patient with hypovolemic shock due to dirrheal disease of less than 1-day duration. Her clinical course was similar to that of severe cholera--she excreted greater than 60 liters of stool and urine in a 4-day period. The etiological agent was a non-enteropathogenic serotype but enterotoxigenic strain of Escherichia coli (063:NM). The patient responded both agglutinating and antitoxic antibodies against this strain and its enterotoxin. An "enteropathogenic serotype," 0111:B4, was also isolated but this finding had no etiological significance.