In a prospective 2-year study, 218 infants aged less than 6 months admitted to a children's hospital in Calcutta with acute diarrhoea and 102 infants (control group) from the out-patient department of the same hospital with similar age, sex and socio-economic composition as the diarrhoea patients, were concurrently investigated for enteropathogenic agents in the faeces. Enteropathogenic Escherichia coli, enterotoxigenic E. coli, and enteroadherent E. coli constituted the 3 types of diarrhoeagenic E. coli most frequently (57·4%) detected in the faeces of diarrhoeic infants either as a sole pathogen or in association with other microorganisms. Other enteropathogens detected were rotavirus, Campylobacter jejuni, Salmonella typhimurium and Aeromonas hydrophila. The frequency of detection of various aetiological agents was 72%; in the remaining 28% an infectious aetiology could not be determined. The study suggests that diarrhoeagenic E. coli plays a major role in the causation of acute diarrhoea in infants aged under 6 months in Calcutta. © 1991.