In vitro and in vivo evolution experiments on Escherichia coli revealed several principles of bacterial adaptation. However, few data are available in the literature describing the behavior of E. coli in its natural environment. We attempted here to study the evolution in the human gut of a commensal dominant E. coli clone ED1a belonging to B2 phylogroup, through a longitudinal genomic study. We sequenced 24 isolates sampled at three different time points within a healthy individual over almost a year. We computed amutation rate of 6.90x10-7 per base per year of the chromosome for E. coli ED1a in healthy human gut. We observed a very limited genomic diversity, and could not detect any evidence of selection contrary to what is observed in experimental evolution over similar length of time. We therefore suggest that ED1a being well adapted to the healthy human gut evolves mostly neutrally with a low effective population size (Ne ≈ 500 – 1700).