We have studied the effects of aging on fecal microbiota composition in the senescence-accelerated prone mice SAMP8 strain. We compared animals 2, 4 and 6 months old. Feces were collected at the end of each period and a genomic study was carried out on fecal DNA using the Illumina MiSeq analyzer. The Shannon diversity index showed similar values along this period and the number of species was neither affected by aging. The most abundant phyla were Firmicutes and Bacteroidetes and did not change with age. However, the phylum Verrucobacteria went up with age, showing a 7-fold increase at 6 months, compared to 2 months old mice. At family level, changes observed between 2 and 6 months of age involved significant increases in Bacteroidaceae (p<0.001) and strong reductions in Lactobacillaceae (p<0.0001) and Prevotellaceae (p<0.05); at genus level, there was a significant reduction in probiotic Lactobacillus. At species level, we observed an age-related reduction in Lactobacillus hayakitensis, a species involved in mucosal immune homeostasis, and in Blautia hansenii, which provides protection against Clostridium difficile infection. Interestingly, aging increases Parabacteroides goldsteiini which is involved in the regulation of the TLR4 pathway. These results support the view that aging results in the proliferation of bacterial species that are associated with the immune deterioration of the gut mucosa.