Résumé : The catabolic ornithine carbamoyltransferase (EC 2.1.3.3) from Pseudomonas aeruginosa, that shows allosteric behaviour, and a mutant version of this enzyme has been crystallized in several different crystal forms. All of these have been characterized by X-ray diffraction methods. A 4.5 Å resolution data set has been collected on a triclinic crystal. Analysis of the data using the self-rotation function shows that 12 monomers associate to form a particle with cubic 23 point group symmetry.