PIP: 48 patients admitted to a rural Bangladesh hospital with dehydration secondary to diarrhea were examined for infection caused by (R.L.A.) reovirus-like agent or (E.T.E.C.) enterotoxigenic Escherichia coli. The diagnosis of R.L.A. infection was established by electron microscopy of stool filtrates and by a 4-fold or greater rise in serum complement-fixing antibodies to the Nebraska calf diarrhea virus. Evidence of infection by heat-labile-toxin producing E.T.E.C. was sought by stool culture and serological testing using the adrenal cell tissue-culture system. Infection by heat-stable-toxin producing E.T.E.C. was sought by stool culture using the infant mouse test. 12 patients, all less than 2 years old, had evidence of R.L.A. infection, accounting for illness in 5% of the 22 patients under 2. None of these 22 had evidence of E.E.T.E.C. infection. R.L.A. diarrhea lasted 5-6 days, often led to serious dehydration, and was associated with vomiting and fever. 11 cases of E.T.E.C. diarrhea were detected, accounting for 56% of the cases of diarrhea in the 18 patients who were more than 10 years old. Diarrhea caused by E.T.E.C. was sudden in onset, shorter in duration, and caused pronounced dehydration. In a community survey, E.T.E.C. was isolated with equal frequency in the stools of control and case family members. Data suggest that E.T.E.C. is a common cause of adult diarrhea in Bangladesh while R.L.A. is a common cause of diarrhea in children.