We have engineered and analyzed a chloroplast mutant of Chlamydomonas reinhardtii that lacks ycf8, the chloroplast open reading frame 8, which is highly conserved in location and predicted amino acid sequence in land plants and C.reinhardtii. The ycf8 sequence was replaced with the aadA cassette which confers resistance to spectinomycin when expressed in the chloroplast. Although the mutant is able to grow phototrophically, photosystem II function and cell growth are impaired under stress conditions such as high light intensity and diminished chloroplast protein synthesis induced by spectinomycin. Use of an antibody generated against the ycf8 product has revealed that this hydrophobic polypeptide is associated with photosystem II, based on its severely reduced levels in various photosystem II-deficient mutants and on its copurification with photosystem II. This protein, therefore, appears to be (i) a novel photosystem II subunit and (ii) required for maintaining optimal photosystem II activity under adverse growth conditions.