A large diarrhea outbreak due to enteroinvasive Escherichia coli (EIEC) serogroup O143 occurring in Houston, Tex., provided the opportunity to investigate aspects of the molecular epidemiology of this and related organisms. This was done by comparing the plasmid patterns and the chromosomal restriction endonuclease digestion patterns by pulsed-field gel electrophoresis (PFGE) of EIEC from the outbreak, other E. coli from the same serogoup (O143), and EIEC isolated from other patients with diarrhea. Among the isolates studied, there was marked restriction fragment length polymorphism. All 3 non-O143 EIEC isolates had very different restriction endonuclease digestion patterns, as did 5 of 5 O143 non-EIEC isolates and 6 of 15 O143 EIEC isolates. Four Houston outbreak O143 EIEC isolates had the same restriction pattern as an O143 EIEC strain isolated 2 months before in Mexico and was nearly identical to another two O143 EIEC Mexican isolates. These related strains also had the same plasmid pattern; however, the presence of only a few plasmid bands, versus the 21 to 30 chromosomal bands seen with PFGE, suggests that plasmid patterns could be a less specific way to distinguish different strains. These results demonstrate that PFGE can distinguish between different E. coli strains of the same serogroup and phenotype. This technique can also identify relatedness within O143 EIEC, and our data suggest the spread of a strain of EIEC from Mexico to Houston, where it caused a large outbreak. PFGE may be useful to study the epidemiology of EIEC.