Approximately 24% of a sample of pathogenic Escherichia coli strains from different serogroups were found to synthesize colicins. Serogroup O55 had an unusually large proportion of such strains (33%). In a sample of 27 O55 isolates, one synthesized a class A colicin (identified as ColE9), five produced class B colicins (three Colla, two, unidentifiable), and three a class A and a class B together.