1. Mitochondria from the nuclear mutant cni-1 have no optically detectable cytochrome aa3 in early log phase growth. These mitochondria have a high level of respiration that is not inhibited by cyanide but is inhibited by salicylhydroxamic acid. They also show a substantial amount of cyanide-sensitive respiration. 2. As cultures of mutant cni-1 age, flux through the hydroxamate-sensitive pathway decreases markedly while flux through the cytochrome chain remains constant. 3. Growth studies with mutant cni-1 indicate that the cytochrome chain in this mutant is more important in supporting growth than the hydroxamate-sensitive pathway. 4. Measurements of the steady-state level of reduction of cytochrome c in mutant cni-1 indicate that the rate-limiting step in the cytochrome chain is at the position occupied by cytochrome oxidase. 5. Electron spin resonance studies with cni-1 mitochondria show normal cytochrome oxidase signals in the g approximately 6 region although there is little or no optically detectable cytochrome aa3.