106 enterotoxigenic Escherichia coli strains from children and adults from many parts of the world were serotyped for O and H antigens. Some O:H types, i.e. O6:H16, 08:H9, O15:H11, O25:H42, O78:H11 and O78:H12, were found repeatedly from different geographical locations. Some of these O:H serotypes were only found rarely among more than 20000 E. coli strains collected over many years from different locations and sources. It is suggested that these special O:H serotypes represent clones which have been selected to the special conditions in the small intestine and selected to carry the plasmids necessary to provoke diarrhoea.