The aerobic flora of 2 groups of children (normal and with malnutrition) with acute diarrhoea was studied, by intubation of the upper and middle small intestine and by stool culture. All the 27 children studied presented bacterial concentrations of 10(5) germs/ml at one or both levels studied. In 9 cases enteropathogen bacteria e were isolated from stools, and in 6 of these they were also found in the small intestine. The results show the elevated incidence of overgrowth of the small intestinal aerobic flora in children with acute diarrhoea. This fact is mentioned as another etiological factor to be taken into consideration in this pathology.