Thermostable cytochrome c-552 of Hydrogenobacter thermophilus was heterologously expressed in the periplasm of Pseudomonas aeruginosa using a fusion gene coding for the P. aeruginosa cytochrome c-551 signal peptide and the H. thermophilus mature cytochrome c-552. The values of the thermal denaturation parameters of periplasmically expressed cytochrome c-552 were identical to those of the authentic protein purified from H. thermophilus.