The three-dimensional structure of the allosteric enzyme aspartate carbamoyltransferase (EC 2.1.3.2) has been refined to a crystallographic R-factor of 0.24 at 2.6-A resolution in the space group P321, where a and b are 122.1 A and c is 142.2 A. This structure is isomorphous to the form of the enzyme complexed to the allosteric inhibitor cytidine triphosphate. All sources of sequence information have been evaluated against the electron density. The corrected amino acid sequences of the catalytic and regulatory proteins have been incorporated in the model, and three regions in the active site are described: (i) near arginine-105, histidine-134, and arginine-167, (ii) near lysine-232 and arginine-229, and (iii) near lysine-83 and lysine-84.