Fimbrial adhesins that mediate attachment to host cells are produced by most virulent Escherichia coli isolates. These virulence factors play an important role in the initial stages of bacterial colonization and also in determination of the host and tissue specificity. Isolates belonging to serotype O78 are known to cause a large variety of clinical syndromes in farm animals and humans and have been shown to produce several types of adherence fimbriae. We studied the fimbrial adhesin from an avian septicemic E. coli isolate of serotype O78. Analysis of the genetic organization of the fac (fimbria of avian E. coli) gene cluster indicates that it belongs to the S-fimbrial adhesin family. Seven open reading frames coding for major and minor structural subunits were identified, and most of them showed a high degree of homology to the corresponding Sfa and Foc determinants. The least-conserved open reading frame was facS, encoding a protein known to play an important role in determining adherence specificity in other S-fimbrial gene clusters.