Isocitrate lyase and malate synthase are specific enzymes of the glyoxylate cycle, used here as glyoxysomal markers. Both enzymes were found in the mitochondrial fraction after organelle fractionation by isopycnic centrifugation. Electron microscopy of this fraction indicated that mitochondria were the only recognizable organelles. Using an immunogold labeling method with anti-(malate synthase) antiserum, the only organelles stained in cells were the mitochondria. These results show that the glyoxylate cycle is present in mitochondria in Euglena.