1. Mitochondria of the obligately aerobic ciliate protozoon, Tetrahymena pyriformis strain ST, are unusual in that they possess a cytochrome oxidase system that does not react with reduced mammalian cytochrome c; the presence of cytochromes a603+a3 is masked in the α-band region of spectra by the broad absorption band of cytochrome a620. 2. Other haemoproteins present include cytochromes b560, b556, c553 and c549. 3. The reaction of reduced cytochrome a3 with CO is reversed by flash photolysis, and in the presence of O2 the subsequent oxidation of this cytochrome is followed by that of cytochrome a603. 4. Cytochromes a620 and b560 also react with CO and with KCN; the latter cytochrome corresponds with that designated cytochrome o by other workers. 5. The contribution of cytochrome a603 to difference spectra is revealed by making use of the fact that it does not react with KCN. 6. Cytochrome a620 is unstable, and its α-absorption band is lost from spectra of mitochondria which have been aged or treated with ultrasound, detergents or organic solvents. 7. Possible pathways of electron transport via the several different terminal oxidases in Tetrahymena mitochondria are proposed.

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