Résumé : The properties of the catabolic ornithine carbamoyltransferase of Pseudomonas fluorescens are compared to those of the anabolic transferase of the same organism. The comparison is also extended to a mutationally modified enzyme which, in contrast to the catabolic enzyme, is able to synthesize citrulline in vivo. All three activities are measured in the direction of citrulline synthesis. The catabolic ornithine carbamoyltransferase and the mutationally modified enzyme have an approximate molecular weight of 400000 while that of the anabolic enzyme is 100000. The catabolic transferase exhibits a number of regulatory properties which are not observed with the anabolic enzyme: Its saturation by carbamyolphosphate is intensively cooperative. Its half saturation concentration is more than one hundred times higher than the Km of the anabolic enzymes; this explains its inability to perform an anabolic function in vivo. Phosphate, a substrate in the physiological direction of the reaction and a low‐energy‐level indicator, acts as an activator of the enzyme; it abolishes the cooperativity of the carbamoylphosphate and increases the capacity of the enzymes. ATP, a product of the catabolic pathway and a high‐energy‐level signal, reduces the capacity of the enzyme. Putrescine, another product of the catabolic pathway, and its analogs, inhibit the enzyme by decreasing its apparent affinity for carbamoylphosphate. The catabolic transferase of the mutant IRC204M1 shares all those properties with the wild‐type enzyme, but, owing to a reduced cooperativity of carbamoylphosphate, it is able to perform an anabolic function at reduced rate. Thus, the comparison of the regulatory properties of the catabolic ornithine carbamoyltransferase and of the modified enzyme provides a kinetic explanation for the inability of the catabolic enzyme to play an anabolic role and this without the need of referring to compartmentation of channeling of the anabolic and catabolic pathways. Copyright © 1972, Wiley Blackwell. All rights reserved