The relative contributions of the dicarboxylic acid and the tricarboxylic acid cycles to the oxidative catabolism of glyoxylate in Escherichia coli K-12 were deduced by analysis of mutant strains that were blocked in the formation of glyoxylate carboligase and of malate synthase G (the "glycolate form" of malate synthase). Mutant strains unable to form malate synthase G were unimpaired in their ability to oxidize glyoxylate. Hence, the dicarboxylic acid cycle does not appear to play an essential role in this process. Organisms blocked in the synthesis of glyoxylate carboligase did not oxidize glyoxylate at a detectable rate, indicating that wild-type organisms convert glyoxylate to acetyl-coenzyme A and oxidize it via the tricarboxylic acid cycle. The foregoing evidence indicates that malate synthase G plays an anaplerotic role during growth with glycolate or acetate as the carbon source. The in vivo activity of malate synthase G was not detectable when the intracellular concentration of acetyl-coenzyme A was low, suggesting that this substrate or a closely related metabolite exerts a sensitive positive control over the enzyme. The synthesis of malate synthase G appears to be induced directly by glycolate which may be formed by a constitutive reduced nicotinamide adenine dinucleotide phosphate-dependent glyoxylate reductase in glyoxylate- or acetate-grown cells.