All proteins involved in the attachment and effacement lesion produced by enteropathogenic Escherichia coli (EPEC) and Shiga-toxin producing E. coli (STEC) are encoded by the locus of enterocyte effacement (LEE). We studied the presence and insertion site of the LEE in different EPEC and STEC strains. In serotypes O119:H6/H-, O55:H6, O55:H7, O142:H6, O111ac:H9/H-, O111ab:H9/H- LEE is inserted downstream of selC as previously described for EPEC O127:H6 and STEC O157:H7. In serotypes O111ac:H8/H- and O26:H11/H- the LEE is inserted in pheU as previously described for STEC O26:H-. However in EPEC from serotype O111ab:H25 the LEE is not inserted in either site suggesting a third insertion site in the K12 chromosome. We also cloned fragments of 2.3 kb and 1.0 kb from the right and left hand sides of the LEE of a O111ac:H- strain and identified additional insertion sequences on these LEE fragments, suggesting that the LEE may be larger and may have undergone more recombination events in these serotypes.