Dihydroorotase (EC 3.5.2.3) catalyzes the reversible cyclization of N-carbamoyl-L-aspartate to L-dihydroorotate in the third step of the pyrimidine-biosynthesis pathway in Bacillus anthracis. A comparison is made between the structures of dihydroorotase from four different organisms, including B. anthracis dihydroorotase, and reveals substantial variations in the active site, dimer interface and overall tertiary structure. These differences demonstrate the utility of exploring multiple structures of a molecular target as expressed from different organisms and how these differences can be exploited for structure-based drug discovery.