Dichelobacter nodosus is a Gram-negative anaerobic bacterium that is the causative organism of footrot in sheep. A D. nodosus locus responsible for a modification of the host lipopolysaccharide (LPS) in Escherichia coli was cloned and sequenced. Genetic studies showed that the modification occurred within the inner-core region of the host LPS, most likely to one or more of the heptose molecules. Antibodies eluted from the modified LPS reacted preferentially with the lipid-A-core region of D. nodosus LPS, suggesting that the cloned epitope was present in this region of the D. nodosus LPS. The gene responsible for the modification, IpsA, potentially encoded a polypeptide of approximately 37 kDa which was highly basic, a characteristic of enzymes which interact with the acidic inner LPS core. The IpsA gene appeared to be arranged in a complex operon with a downstream gene, prfC, which encoded a protein with similarity to E. coli peptide-chain release factor 3.