Amicronucleate (strain W) and micronucleate (strain HAM 3)Tetrahymena pyriformishave been studied by light and electron microscopy. Numerous interphase and individually‐sectioned organisms in division stages have been examined. In macronuclei of the amicronucleate organisms (strain W) the time of division is characterized by a rather marked chromatin condensation, by morphological changes in nucleoli, and by the presence of dense granules. In macro‐nuclei of the micronucleus‐possessing organisms (strain HAM 3) little alteration of the interphase morphology is observed at division. Mitochondrial changes and an increase in the amount and complexity of the endoplasmic reticulum are observed in division in the amicronucleate strain. These results show that macronuclear division is more complex than a simple “pinching‐in‐two” and that loss of the micronucleus may alter the mitotic events of the