Two low molecular weight subunits (ca. 4 kDa) have been identified in highly purified preparations of cytochrome b6f from the green unicellular alga Chlamydomonas reinhardtii. The N-terminus of the first one is blocked. It is synthesized in the chloroplast. It cross reacts with an antiserum raised against a synthetic peptide with the C-terminal sequence of the predicted product of the chloroplast gene petG. The homologue of this protein had been previously identified by other authors in the b6f complex from maize. The N-terminal sequence of the second subunit does not correspond to any known protein. This polypeptide, provisionally named petX, is synthesized in the cytosol. An antiserum has been raised against the corresponding synthetic peptide. Immunoblotting experiments show that neither petG nor petX are present in thylakoid membranes from a b6f-less strain of C. reinhardtii.