Colonization of the gastrointestinal tract in newborns delivered by Cesarean section occurs during the first days of life by bacteria provided by the environment. It is more rapid in breast-fed infants than in bottle-fed babies. The intestinal flora is more diversified if the infants receive formula feeding. The first bacteria encountered are facultative anaerobes; they remain predominant during the first two weeks of life. In comparison to vaginal delivery, there were low levels of strict anaerobes after Cesarean section; members of the Bacteroides fragilis group were still completely lacking after 14 days of life and Bifidobacteria were only isolated sporadically.