To investigate if the characteristics of human intestinal Escherichia coli are changing with the environment of the host, we studied intestinal E. coli from subjects having recently migrated from a temperate to a tropical area. We determined the phylogenetic group, the prevalence of the antibiotic resistance, the presence of integrons and the strain diversity in faecal isolates from 25 subjects originally from metropolitan France and expatriated to French Guyana. These characteristics were compared with those of 25 previously studied Wayampi Amerindian natives of French Guyana and from 25 metropolitan French residents. The three groups of subjects were matched for age and sex, had not taken antibiotics for at least 1 month, nor had been hospitalized within the past year. In all, the characteristics of intestinal E. coli from Expatriates were intermediate between those found in residents from metropolitan France and those found in natives of French Guyana. Prevalence of carriage of resistant Gram-negative bacteria in Expatriates was intermediate between French residents and Wayampi as were the prevalence of integrons in E. coli (12.3% versus 16.3% and 7.8% respectively), and the intra-host diversity of E. coli (2.3 strains/subject versus 1.9 and 3.1, respectively); lastly, in Expatriates, the prevalence of carriage of phylogenetic group B2 strains was lower than in French residents (16% versus 56%, P = 0.005), while carriage of phylogenetic group A strains was lower than in Wayampi (56% versus 88%, P = 0.03). Our results suggest that the composition of the commensal intestinal flora of humans is not static but changes dynamically in response to new environmental conditions.