Thirty-nine Escherichia coli isolates from broiler chickens with cellulitis were serotyped and analyzed for clonal relationships by multilocus enzyme electrophoresis. The isolates were further characterized with respect to hemagglutination (HA); serum resistance; antibiotic susceptibility; production of aerobactin, colicin V, and hemolysin; expression of K1 or K5 capsule; sensitivity to cloacin DF13 after treatment with diphenylamine; expression of iron-regulated outer membrane proteins; and virulence in 1-day-old chickens. In addition, the isolates were examined for the presence of DNA sequences related to F1A (fim) and P (pap) fimbriae, aerobactin synthesis (iuc) and transport (iut), hemolysin operon hly, and TraT lipoprotein-induced serum resistance (traT). Only 38.4% of the isolates were typeable with standard O antisera, and of these, serogroups O25 and O78 were the most frequently observed. Multilocus enzyme electrophoresis, based on 20 enzymes, resolved 17 electrophoretic types, forming seven clusters. Isolates from four of these clusters fell into E. coli clone complexes that have been previously reported to be commonly associated with avian colibacillosis. All isolates expressed two to five iron-regulated outer membrane proteins, were resistant to serum and cloacin DF13, and possessed DNA sequences homologous to fim and iuc/iut. Most isolates (72%) were positive for traT, and a majority produced colicin V and aerobactin (92 and 82%, respectively). Assays for the presence of fim and pap DNA sequences, for HA, and for virulence gave variable results but suggest that cellulitis isolates may express F1A and/or other mannose-resistant HA fimbriae different from P and may be virulent in 1-day-old chickens. Our results support the hypothesis that cellulitis in broilers in many cases is caused by E. coli clones identical to other pathogenic avian E. coli strains. Certain clones may be specific to cellulitis, because 25% of the isolates tested belong to clusters not related to known clone complexes.