Highly purified preparations of cytochrome b6 f complex from the unicellar freshwater alga Chlamydomonas reinhardtii contain about 1 molecule of chlorophyll a/cytochrome f. Several lines of evidence indicate that the chlorophyll is an authentic component of the complex rather than a contaminant. In particular, (i) the stoichiometry is constant; (ii) the chlorophyll is associated with the complex at a specific binding site, as evidenced by resonance Raman spectroscopy; (iii) it does not originate from free chlorophyll released from thylakoid membranes upon solubilization; and (iv) its rate of exchange with free, radioactive chlorophyll a is extremely slow (weeks). Some of the putative functional roles for a chlorophyll in the b6f complex are experimentally ruled out, and its possible evolutionary origin is briefly discussed.