A plasmid for expression of the thermophilic bacterial cytochrome c-551 in Bacillus subtilis was constructed, The transformed cells of B, subtilis 1012 looked slightly pink, and cytochrome c-551 was found in the membrane fraction in amounts as large as 2.5 nmol per mg membrane protein. The cytochrome was extracted with cholate, and then purified by precipitation with polyethylene glycol, ion-exchange chromatography and,eel filtration, Analysis of the post-translational modification of the cytochrome showed that similar events, such as signal peptide cleavage, and modification of the new N-terminal cysteine residue with diacylated glycerol, took place as in the case of thermophilic bacilli, but the blocking of the N-terminus was incomplete in B, subtilis.