To determine if there is any change with age in the distribution of Bi dobacterium and Enterobacteriaceae species in human intestinal micro ora, strains were isolated from a total of 54 samples of human faeces (15 children, 3-15 years old; 17 adults, 30-46 years old; 22 elderly, 69-89 years old). The bi dobacteria were identi ed at species level using a numerical phenotypic identi cation system developed on the Internet (http:::kounou.lille.inra.fr) and DNA-DNA hyridization. The enterobacteria were identi ed using the API 20E system (Biomérieux, France). Escherichia coli was present in 93% of samples. Enterobacter and Klebsiella species were more frequently found in children while Proteus and Pro×idencia species were typically found in the elderly. The species Bi dobacterium longum was the most frequently species isolated in children and adults, whereas Bi dobacterium adolescentis was the most encountered species in the elderly. The distribution of B. adolescentis was signi cantly split between the children and the elderly age groups. There is a noticeable trend whereby Bi dobacterium pseudocatenulatu m occurs more frequently in children than in the elderly. The species Bi dobacterium infantis, Bi dobacterium gallicum and Bi dobacterium angulatum have never been isolated, while Bi dobacterium bre×e has only been isolated on one occasion.