Mitochondria and glyoxysomes were isolated from scutella of maize (Zea mays L.) by density gradient centrifugation. Citrate synthetase was partly solubilized from the organelles by sonication. The sonicated organelle suspensions were centrifuged at high speed, and the supernatants were used as enzyme preparations without further purification. The enzymes of the two organelles differ in all properties examined (pH activity curve, Km for substrates, elution volume on Sephadex G-100, mobility on starch gel at pH 7). Both enzymes are inhibited by ATP, but the inhibition is stronger for the mitochondrial enzyme. The inhibition is competitive for the mitochondrial enzyme and noncompetitive for the glyoxysomal enzyme. The glyoxysomal, but not the mitochondrial enzyme, is inhibited 40% with 1 mm ADP and cytidine triphosphate.