Cell-free extract supernatant fluids of Pseudomonas aeruginosa were shown to lack malic dehydrogenase but possess a nicotinamide adenine dinucleotide (NAD) - or NAD phosphate (NADP)-dependent enzymatic activity, with properties suggesting a malic enzyme (malate + NAD (NADP))→ pyruvate + reduced NAD (NADH) (reduced NADP [NADPH] + CO2), is agreement with earlier findings. This was confirmed by determining the nature and stoichiometry of the reaction products. Differences in heat stability and partial purification of these activities demonstrated the existence of two malic enzymes, one specific for NAD and other for NADP. Both enzymes require bivalent metal cations for activity, Mn2+ being more effective than Mg2+. The NADP-dependent enzyme is activated by K+ and low concentrations of NH4+. Both reactions are reversibles, as shown by incubation with pyruvate, CO2, NADH, or NADPH and Mn2+. The molecular weights of the enzymes were estimated by gel filtration (270,000 for the NAD enzyme and 68,000 for the NADP enzyme) and by sucrose density gradient centrifugation (about 200,000 and 90,000 respectively).