The rfb region of Yersinia pseudotuberculosis serogroup IIA has been cloned and expression of O antigen in Escherichia coli K12 was demonstrated. Transposon mutagenesis analysis confined the DNA region required for O antigen expression to a 19.3 kb fragment, and the O antigen expressed was visualized by SDS-PAGE and silver staining. Southern hybridization analysis demonstrated significant levels of similarity between the Yersinia rfb region and the 3,6-dideoxyhexose pathway genes rfbF and rfbG, previously isolated from Salmonella enterica LT2, but no similarity to the abequose synthase gene rfbJ of the same strain or the paratose synthase gene rfbS isolated from S. enterica Ty2. The evolutionary relationship between the abequose biosynthetic genes of the two species of Salmonella and Yersinia is discussed.