A pathogenicity locus in Xanthomonas campestris pv. campestris has been shown to comprise two genes which mediate biosynthesis of the bacterial lipopolysaccharide (LPS) but not extracellular polysaccharide. Mutants with Tn5 insertions in either gene showed alterations in the electrophoretic patterns of both water-soluble and phenol-soluble LPS forms, which suggested defects in the biosynthesis of the core oligosaccharide component. On gel chromatography, core oligosaccharides of the mutants were of apparently lower molecular weight than those from the wild type. Furthermore, the content of mannose and glucose, sugars characteristic of the core oligosaccharide, were significantly lower in the water-soluble LPS of the mutants. Because of their role in LPS core biosynthesis, the two genes were called rfaX and rfaY. rfaX mutants show altered behavior in a range of host and non-host plants such that the number of recoverable bacteria drop within the first 24 h after inoculation. In contrast, the behavior of rfaY mutants only differed from the wild type in Datura, a non-host plant in which the growth of the wild type is severely attenuated. The predicted protein RfaY showed significant sequence homology to a sub-family of RNA polymerase sigma factors which are involved in extracytoplasmic functions.