Nature achieves the epimerization of carbohydrates by a variety of chemical routes. One common route is that performed by the class of enzyme defined by dTDP-6-deoxy-D-xylo-4-hexulose 3,5-epimerase (RmlC) from the rhamnose pathway. Earlier studies failed to identify the key residues in catalysis. We report the 1.3 A structure of RmlC from Streptococcus suis type 2 and its complexes with dTDP-D-glucose and dTDP-D-xylose. The streptococcal RmlC enzymes belong to a separate subgroup, sharing only 25% identity with RmlC from other bacteria, yet the S. suis enzyme has similar kinetic properties and structure to other RmlC enzymes. Structure, sequence alignment, and mutational analysis have now allowed reliable identification of the catalytic residues and their roles.