SUMMARYThe faecal flora of 55 children (aged 8 days to 8 years) and 16 adults was determined. All the children were artificially fed from birth. The faecal flora of the youngest age group was generally less complex and less predictable than that of adults. Some bacterial groups commonly found in adult stools, for example bacilli, lactobacilli and yeasts, were rarely found in the youngest infants. Most of the changes towards the adult pattern took place between 4 and 12 months. The faecal flora of children aged 1-4 years generally resembled that of adults, although lactobacilli were still infrequently isolated.