Escherichia coli O157:H7 is a source of foodborne illness, causing diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome. E. coli O157:H7 secretes, via the etp type II secretion system, a metalloprotease, StcE, that specifically cleaves the serpin C1 esterase inhibitor. We determined by hybridization techniques the prevalence of stcE and etpD, a type II secretion gene, among diarrheagenic E. coli strains. stcE and etpD are ubiquitous among the O157:H7 serotype and are found in some enteropathogenic E. coli O55:H7 strains but are absent from other diarrheagenic E. coli. stcE was acquired on a large plasmid early in the evolution of E. coli O157:H7, before the inheritance of the Shiga toxin prophage. Other plasmidborne virulence factors, such as ehxA, katP, and espP, were acquired later by the enterohemorrhagic E. coli 1 complex in a stepwise manner. These data refine the sequential model of E. coli O157:H7 evolution proposed elsewhere.