Sites of tissue colonization and persistence of highly and weakly virulent Escherichia coli were determined in 3-week-old gnotobiotic turkeys. Both strains of E. coli colonized respiratory and intestinal tracts. E. coli persisted in large numbers in crop, ileum, ceca, colon, and lung. Translocation of E. coli from intestine to liver occurred early in infection and was inhibited by a concurrent Streptococcus faecalis infection. Persistence of E. coli in liver was positively correlated with virulence. This study indicates that colonization and persistance of E. coli in intestine and lung of gnotobiotic turkeys is not correlated with virulence;Highly and weakly virulent strains of E. coli were examined for their ability to express type 1 pili in the intestinal tract of gnotobiotic turkeys. Non-piliated forms of both strains were more numerous than piliated forms in caecal and colonic contents. Mucus overlying cecal and colonic epithelium had large numbers of piliated forms of both strains. Organisms were seldom intimately associated with intestinal epithelium. Serum agglutinating antibody titers to type 1 pili were detected in turkeys inoculated with either strain. This study indicates that type 1 pili are expressed in the intestinal tract of turkeys by E. coli strains of high and low virulence. In addition, type 1 pili do not appear to mediate in vivo adherence of E. coli to intestinal epithelium of turkeys. Type 1 pili may promote bacterial trapping within intestinal mucus or adherence to specific receptors in mucus;Pathologic changes of cloacal bursae were studied in 3-week-old gnotobiotic turkeys inoculated orally with E. coli strains of high and low virulence. Granulocytic inflammation of bursae developed early and diminished by 8 days post-inoculation of turkeys given either strain. Pyogranulomatous bursitis with follicular necrosis developed by 5 days postinoculation and increased in severity with time in turkeys inoculated with the weakly virulent strain. Bacteria were found within necrotic centers of follicular abscesses, in phagosomes of multinucleate giant cells and macrophages within follicular medullae and within follicular pads. These findings suggest that E. coli may pass through the follicular-associated epithelium of the cloacal bursa, replicate within follicular medullae and produce extensive follicular necrosis.