Mutants devoid of malate dehydrogenase activity have been isolated in Escherichia coli K-12. They do not possess detectable malate dehydrogenase when grown aerobically or anaerobically on glucose as sole carbon source. All mutants revert spontaneously; a few partial revertants have been found with a malate dehydrogenase exhibiting altered electrophoretic mobility. Therefore, only one such enzyme appears to exist in the strains examined. No evidence could be obtained for the presence of a malate dehydrogenase not linked to nicotinamide adenine dinucleotide. Mutants deficient in both malate dehydrogenase and phosphoenol pyruvate carboxylase activities will grow anaerobically on minimal glucose plus succinate medium; also, malate dehydrogenase mutants do not require succinate for anaerobic growth on glucose. The anaerobic pathway oxaloacetate to succinate or succinate to aspartate appears to be accomplished by aspartase. Malate dehydrogenase is coded for by a locus somewhere relatively near the histidine operon, i.e., a different chromosomal location than that known for other citric acid cycle enzymes.