The faecal flora of 29 healthy infants and young children was compared with that of 49 children of similar age and socio-ecomonic status with acute gastroenteritis. In the healthy children the most common organisms in the faeces were bifidobacteria, veillonellae, enterobacteria and enterodocci with anaerobes outnumbering aerobes. Most members of the noraml faecal flora were present in the diarrhoeal stools, but anaerobes were signigicantly reduced in number and enterobacteria were significantly increased, thereby altering the ratio of anaerobes to aerobes. The alterations in the flora were not related to the nature of the aetiological agent or to the severity of the diarrhoea. The changes appeared to be a direct result of the altered colonic environment produced by the diarrhoeal state. In 13 of the 28 patients from whom bacterial pathogens were isolated, the pathogens were the predominant faecal organsims.