Three mutant strains of Neurospora crassa, previously characterized as cytochrome c oxidase deficient, were found to possess this respiratory enzyme. The mitochondrial cytochrome contents of logarithmic phase cells of wild type (74A) and the respiration-deficient strains mi-3, cya-4-23, and cya-5-34 were investigated with low temperature difference spectrophotometry; two of the strains were found to possess cytochrome a. Cytochrome c oxidase catalytic activity measurements performed on isolated mitochondria showed that all three mutant strains contained significant amounts of active enzyme which was subsequently found to be active in vivo. However, activity of the cyanide-insensitive, alternate electron transport pathway was responsible for most of the oxygen consumed by the logarithmic phase cells of these mutant strains. Electrophoretic analysis of the polypeptide subunits of cytochrome c oxidase radiolabeled in vivo during logarithmic phase growth showed that, in contrast to previous reports, the respiration-deficient strains appeared to contain all seven subunits of the enzyme.