A psychrophilic marine Pseudomonas was found to contain phosphoenolpyruvate (PEP) carboxylase and an adenosine triphosphate-linked PEP carboxykinase. Some properties of these CO2-fixing enzymes were compared with those homologous enzymes from the terrestrial mesophile Enterobacter cloacae. The PEP carboxylases from both organisms were activated by acetyl-coenzyme A (CoA) and inhibited by l-aspartate. The enzyme from Pseudomonas was less dependent on the presence of the activator, but maximal activation was attained at acetyl-CoA concentrations much lower (50 μm) than those required to saturate the enzyme from E. cloacae. In both cases the main effect of acetyl-CoA was to decrease the Km value for PEP. The activity of PEP carboxylase from Pseudomonas was only slightly inhibited by NaCl, KCl, or NH4Cl up to 100 mm, whereas the enzyme from E. cloacae was inhibited by about 70% under similar experimental conditions. Both PEP carboxylase and PEP carboxykinase from Pseudomonas showed considerably lower thermal stability than their counterparts from E. cloacae. Our results suggest that the CO2-fixing enzymes from a marine Pseudomonas and E. cloacae are similar in nature and regulation, but they differ in properties related to the peculiar conditions of the marine environment.