The mitochondrial cytochrome aa3 and b deficiencies of the [poky] cytoplasmic mutant of Neurospora crassa are partially suppressed by mutant alleles of any one of six nuclear genes, namely sup-1, sup-3, sup-4, sup-5, sup-10 and sup-14. The suppressor-induced increases in the concentration of both cytochromes are detected in the mitochondria from exponentially growing [poky] cultures, and, thus, are clearly distinguishable from the age-dependent changes in the cytochrome system that occur in cultures that approach, or have reached, the stationary phase of growth. The relative amounts of mitochondrial cytochromes aa3 and b show a direct correlation with the relative efficiency of the various sup genes as suppressors of the slow-growth phenotype of [poky]. Since [poky] is defective in mitochondrial protein synthesis due to a lack of 30 S mitochondrial ribosomal subunits, it is proposed that the six suppressors promote the assembly of functional mitochondrial ribosomes.