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Mitochondria isolated from protoplasts of conidia, young germinated conidia, and branched mycelia of Neurospora crassa, have an active oxidative phosphorylation system which showed no variation in the three stages of development studied ; succinate dehydrogenase also did not alter. Cytochrome oxidase activity showed a marked increase at the time of conidial germination; this was not maintained during further growth. An electron microscope study of the conidia showed them to have complex mitochondria analogous to those seen in older mycelial hyphae. A large accumulation of lipid material and extensive vacuolization was observed in these conidia. A rudimentary reticular system with numerous ribosomes lying free in this membrane system were observed. Macroconidia were usually multinucleate; up to 10 nuclei were observed. The outer conidial wall was completely digested by snail extract (Helix pomatia digestive juice) and was structurally identical with that of the hyphae as seen with the fixation and embedding conditions used.