The PsbP protein is an extrinsic component of photosystem II that together with PsbO and PsbQ forms the thylakoid lumenal part of the oxygen-evolving complex in higher plants. In addition to PsbP, the thylakoid lumen contains two PsbP-like proteins (PPLs) and six PsbP-domain proteins (PPDs). While the functions of the PsbP-like proteins PPL1 and PPL2 are currently under investigation, the function of the PsbP-domain proteins still remains completely unknown. PPD6 is unique among the PsbP family of proteins in that it contains a conserved disulfide bond which can be reduced in vitro by thioredoxin. The crystal structure determination of the PPD6 protein has been initiated in order to elucidate its function and to gain deeper insights into redox-regulation pathways in the thylakoid lumen. PPD6 has been expressed, purified and crystallized and preliminary X-ray diffraction data have been collected. The crystals belonged to space group P2(1), with unit-cell parameters a = 47.0, b = 64.3, c = 62.0 Å, β = 94.2°, and diffracted to a maximum d-spacing of 2.1 Å.