We have investigated the photosystem-II organization in differentiating-bundle-sheath cells of the three malate dehydrogenase (oxaloacetate decarboxylating) (NADP+)-type C4 species maize, Sorghum and Pennisetum. Using a set of nine different antisera raised against individual subunits of photosystem-II, we demonstrate that photosystem-II components constitute a substantial part of the thylakoid membranes of young bundle-sheath chloroplasts. The abundance of subunits of the photosystem-II core, i.e. the 47-and 43-kDa chlorophyll-a-binding proteins, polypeptides D1 and D2, cytochrome b559, and the 34-kDa polypeptide, varies with the developmental state of the plant. However, the levels of the 23-kDa, 16-kDa and 10-kDa extrinsic polypeptides of the water-oxidation complex are drastically reduced in bundle-sheath chloroplasts of all three species analyzed, regardless of their state of differentiation. The reduction in protein abundance is also reflected at the transcript level: only traces of the nuclear-encoded mRNAs are found in differentiating bundle-sheath cells of Sorghum, suggesting that the transcription of these genes has been switched off. Our data are compatible with the idea that the water-oxidation complex is a prime site for initiating or maintaining the process leading to photosystem-II depletion during differentiation of bundle-sheath cells.