1) Eighty‐seven breast‐fed and 95 bottle‐fed infants were examined for differences inEscherichia coliserogroup pattern. 2) The serogroup composition was less complex in breast‐fed infants. Fewer serogroups were found in the individual breast‐fed child. However, with few exceptions, the same O groups were found in the two groups of infants. The prevalent O groups were among those also found most frequently in faeces from adults. 06 strains were more common in the breast‐fed group, while 07 and 016 were not at all found in that group. 3) A special search forE. colistrains with K1 antigen showed that this antigen predominated in the bottle‐fed infants. 4)Klebsiellaand otherEnterobacteriaceaewere more frequently found among bottle‐fed infants. 5) It is concluded that the intestinal milieu exerts a selective pressure which may result in a selection of different serotypes in two different intestinal milieus such as