In order to assign NMR signals, conditions for the specific labeling of cytochrome $c_3$ of D. vulgaris Miyazaki F through the culture in a minimal medium were established. Phenylalanine residue was specifically deuterated at more than 85% efficiency. Cytochrome $c_3$ has two phenylalanine residues. The signals of one phenylalane were missing and this was tentatively assigned to Phe20.