A mutant of Salmonella typhimurium LT2 was found to be unable to convert cytidine diphospho-4-keto-6-deoxy-d-glucose into cytidine diphosphoabequose. The mutation maps in the rfb gene cluster, which is known to be involved in the biosynthesis of the peripheral, "O side-chain" portion of cell wall lipopolysaccharide. In spite of the fact that, in the O side chains, abequose is not a part of the main chain but occurs as short branches, the mutant appears to be unable to polymerize oligosaccharide "repeat units" into long O side chains. The following evidence indicates that this failure is the result of the absence of cytidine diphosphoabequose rather than that of a superimposed second mutation in other genes of the rfb cluster. (i) The mutant does not behave like a multisite mutant in genetic crosses, and it gives rise, at a high frequency, to "revertants" where the ability to synthesize cytidine diphosphoabequose and the ability to synthesize normal lipopolysaccharide with O side chains are both restored. (ii) The mutant strain has normal levels of activity of all of the other enzymes known to be involved in O side-chain synthesis, except that the levels of several enzymes were lowered by about 30% owing to the polarity effect of the mutation. That the lowering of these enzymes is not responsible for the failure of the mutant to synthesize O side chains is clear from the fact that there were revertants which had regained some ability to synthesize abequose but still had lowered levels of these other enzymes, and that this type of revertant produced lipopolysaccharide with considerable amounts of O side chains.