Orotate phosphoribosyltransferase (OPRTase) is involved in the de novo biosynthesis of pyrimidine nucleotides by catalyzing the Mg2+ dependent formation of orotidine 5‘-monophosphate (OMP), from which uridine 5‘-monophosphate is synthesized. Crystal structure of OPRTase from Bacillus anthracis str. 'Ames Ancestor' has been solved and refined to 1.75 Å resolution with a single copy of the protein per asymmetric unit. The structure resembles the structure of the OPRTase from Streptococcus mutans, which was used as a molecular replacement model. Crystal packing reveals that the B. anthracis OPRTase is a homodimer with a total buried surface area of 1950 Å. In the dimer, active site of each chain is at oligomerization interface. The residues Thr71 through Gly75 and Arg94 through Asn104 are structurally disordered.