Early investigations on arginine biosynthesis brought to light basic features of metabolic regulation. The most significant advances of the last 10 to 15 years concern the arginine repressor, its structure and mode of action in bothE. coliandSalmonella typhimurium, the sequence analysis of allargstructural genes inE. coliandSalmonella typhimurium, the resulting evolutionary inferences, and the dual regulation of thecarAB operon. This review provides an overall picture of the pathways, their interconnections, the regulatory circuits involved, and the resulting interferences between arginine and polyamine biosynthesis. Carbamoylphosphate is a precursor common to arginine and the pyrimidines. In bothEscherichia coliandSalmonella entericaserovar Typhimurium, it is produced by a single synthetase, carbamoylphosphate synthetase (CPSase), with glutamine as the physiological amino group donor. This situation contrasts with the existence of separate enzymes specific for arginine and pyrimidine biosynthesis inBacillus subtilisand fungi. Polyamine biosynthesis has been particularly well studied inE. coli, and the cognate genes have been identified in theSalmonellagenome as well, including those involved in transport functions. The review summarizes what is known about the enzymes involved in the arginine pathway ofE. coliandS. entericaserovar Typhimurium; homologous genes were identified in both organisms, exceptargF(encoding a supplementary OTCase), which is lacking inSalmonella. Several examples of putative enzyme recruitment (homologous enzymes performing analogous functions) are also presented.