Saccharomyces cerevisiae cells grown under strict anaerobiosis possess mitochondria-like particles. These particles are tentatively designated as promitochondria. They lack a respiratory chain but still contain oligomycin-sensitive adenosine triphosphatase (F1), “structural protein,” and mitochondrial deoxyribonucleic acid (density 1.685 g/cc), all of which are indistinguishable from their counterparts in aerobic yeast mitochondria. Promitochondria from anaerobically grown cells of the cytoplasmic “petite” mutant exhibit oligomycin- insensitive F1and thus differ from the corresponding particles of the wild-type strain. Promitochondria are present in anaerobic yeast cells regardless of whether these are grown in the presence or the absence of Tween 80 and ergosterol. These findings suggest that anaerobic growth of S. cerevisiae leads to a dedifferentiation, rather than a complete loss, of the mitochondrial organelles. © 1969, American Chemical Society. All rights reserved.