Cytochrome c oxidase in the respiratory chain of bacteria and mitochondria couples the reduction of molecular oxygen to form water with the generation of a transmembrane proton gradient. Bacillus subtilis has two heme A-containing heme–copper oxidases: the menaquinol oxidase cytochrome aa3 and the cytochrome c oxidase cytochrome caa3. By screening three collections of mutants for defective cytochrome c oxidase, we found the genes for two, new membrane-bound assembly factors in B. subtilis: ytkA and yozB (renamed ctaK and ctaM, respectively). CtaK is a lipoprotein without sequence similarity to any protein of known function. We show that CtaK functions together with Sco1 (YpmQ) in a pathway, leading to the assembly of the CuA center in cytochrome caa3 and seems to be a functional analogue to proteins of the periplasmic CuA chaperone family (PCuAC). CtaM is required for the activity of both cytochrome caa3 and cytochrome aa3 and dispensable for the insertion of heme A into these oxidases. The orthologous Bacillus anthracis protein and the distantly related Staphylococcus aureus CtaM complemented CtaM deficiency in B. subtilis, establishing a common function of CtaM in these bacteria. As the overall result of our work, 12 different proteins are known to function in the biosynthesis of cytochrome c oxidase in B. subtilis.