Stool samples from 196 healthy infants less than ten days of age were cultured to delineate bacterial colonization patterns of the intestine. By one week of age, isolation rates of Bacteroides fragilis and other anaerobic bacteria in term infants following vaginal delivery and formula feeding approach that of adults. Differences in gestational age, type of delivery, and type of feeding are associated with significantly different colonization patterns of anaerobic and aerobic bacteria in the first week after birth.