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The human gut microbiota changes in old age, both in structure and function, and these changes have been linked to diet, health and well-being. Here, we present comparative metagenomic data from the fecal microbiota of 29 healthy young subjects (20-35 y) and 32 healthy older subjects (60-75 y). The analysis was carried out using bacterial tag encoded FLX amplicon pyrosequencing generating a 520bp sequence targeting V1, V2 and V3 regions located on the 16S rRNA gene. We observed statistically significant (p-value< 0.05) differences in faecal microbiota community structure between young and older subjects using non-parametric estimators. At the phylum level, Bacteroidetes and Firmicutes showed the most striking differences, with Bacteroidetes higher in the older subjects (25.4 and 16.7% in old and young respectively), and Firmicutes higher in the young group (80.1 and 69.4% in young and old respectively). Smaller, but significant, differences were also apparent in the Proteobacterium and Tenericutes phyla, which were both higher in the older subjects. Clostridiales, and in particular, Lachnospiraceae and Ruminococcaceae families, were present in significantly higher numbers in stools of young subjects compared with older subjects. Our observations confirm that there are substantial changes in the gut microbiota of young and older adults, and suggest that these are indicative of a lowering of Firmicutes, in particular Clostridiales population, that are …