The quantitative changes in mitochondria and cytochromes during transition of Saccharomyces cerevisiae from one steady state to another, while growing in continuous culture under controlled environmental conditions, were followed. No Mitochondria, or mitochondria like structures, were detectable in electron micrographs of permanganate‐fixed anaerobic cells. Microaerobiosis (3μM dissolved oxygen) was sufficient to visualize mitochondrial profiles and induce cytochromes and their sections had a reduced number of mitochondrial profiles compared with cells grown in limiting glucose. In the presence of ergosterol and Tween 80 mitochondriogenesis, whether induced by aerobiosis or glucose limitation, involved enhanced definition of crystal and outer mitochondrial membranes and increased number of profiles. Where membrane formation was limited, by the absence of aerobiosis involved eytochrome induction and profile visualization, but limited profile Proliferation; the adapted cells consequently contained fewer, but more eytochrome‐enriched, mitochondria than cells adapted in the presence of ergosterol and Tween 80. Increase in dissolved oxygen from 3μM to 52μM further enhanced membrane definition and increased the size, but not the number, of mitochondrial profiles. Evidence, obtained by measurement of eytochrome concentration per unit mitochondrial volume and per unit crystal area, support the concept that mitochondriogensis and cytochrome synthesis are not synchronized process and that cytochromes are added to or depleted from the mitochondrial cristae in response to culture conditions.