ABSTRACTCochlonema odontosperma, a zoopagalean fungus endoparasitic in terricolous amoebae, was examined using ultrathin sections. Conspicuous cellular components in the young thallus were many nuclei (ca1μm in diam), electron-dense vesicles (ca0.5μm) and small vacuoles (0.2-1.0μm). In older thalli there was usually one or more large vacuoles and a number of small vacuoles. Both the large and small vacuoles were often connected by a number of electron-dense vesicles. With vacuole expansion, the protoplasm migrated into the conidiogenous and zygosporangial hyphae outside the thallus.The conidia contained one or more nuclei and several vesicles (ca0.3μm). The latter tended to accumulate beneath the conidial cell wall at protuberances which appeared as bumps on the conidia under the light microscope.