Malate dehydrogenase (decarboxylating) from Tritrichomonas foetus hydrogenosomes was purified close to homogeneity by a combination of differential centrifugation, zwitterionic detergent solubilization, Red-Sepharose chromatography and anion-exchange chromatography. The enzyme with apparent subunit size of 59 kDa and native molecular mass of 308 kDa utilized NAD+ preferentially to NADP+ as a cofactor and required Mn2+ or Mg2+ for its activity. Affinity curves for malate and coenzymes were hyperbolic. Km for malate was 100 μM and 458 μM in the presence of NAD+ and NADP+, respectively. Km for NAD+ and for NADP+ in the presence of malate was 18 μM and 207 μM, respectively. The enzyme is proposed to be a tetramer with a possible physiological role in the maintenance of an appropriate NAD+/NADH ratio in hydrogenosomes.