Potassium permanganate fixed conidia and hyphae ofNeurospora crassawere studied with the electron microscope. The cell wall comprises an outer friable electrondense portion and an inner electron-light region where the fibers seem to be longitudinally oriented. The endoplasmic reticulum is sparse and discontinuous in the conidium but a well developed lamellar system is present in the mycelium; in some instances the ends of the reticulum enlarge to form vesicles. Rounded nuclei are observed both in the hyphae and the conidia, but in the former elongated nuclei also occur. In one case, migrating elongated nuclei in the hyphae are seen to squeeze through a septal pore. In properly fixed material, nucleoplasm is differentiated to electron-dense and electron-light areas. Well defined nuclear envelopes, nuclear pores and continuities between endoplasmic reticulum and nuclear envelopes also are observed. Mitochondria show variations in form, size and structure. Some of the mitochondria eventually give rise to small vacuoles by the disintegration of cristae. These findings are discussed in the light of related reports in the literature.