Electron microscopy of thin sections ofCrithidia fasciculata showed cytoplasmic organelles 0.2–0.8 μm in diameter containing a finely granular matrix limited by a single membrane. Utilization of cytochemical techniques (DAB) for endogenous catalase revealed pronounced deposition of a highly electron-opaque material in these organelles. Reaction of cells with DAB was completely inhibited by 0.02 M 3-amino-1,2,4-triazole. It is concluded that this organelle is a microbody on the basis of cytological and cytochemical similarities with microbodies from plant and other animal cells.