Cell-free extracts ofXanthomonas phaseolicontain the individual enzymes of the tricarboxylic acid cycle, and it is suggested that this is the main pathway for the terminal oxidation of carbohydrate in this organism.X.phaseolican grow on a medium containing acetate as the sole source of carbon. Cell-free extracts of such acetate-grown organisms contain the enzymes of the glyoxylate cycle, and it is concluded that the operation of this cycle permits the initial stages of synthesis of complex cell material from acetate at a rate sufficiently high to account for the observed rate of growth on the acetate medium. The two enzymes required to modify a tricarboxylic acid cycle into a glyoxylate cycle are present in very small amounts (malate synthetase) or absent entirely (isocitritase) in extracts of glucose-grownX.phaseoli.