The 33kDa protein of photosystem II (PSII) is common to all of the organisms and totally exchangeable in binding to PSII from various different organisms (Enami et al. , 2000, Plant Cell Physiol. 41, 1354-1364). The amino acid sequence of the 33kDa protein showed a relatively high homology above 40% from cyanobacteria to higher plants. Thus, the structure of the 33kDa protein has been considered to be conserved during evolution from cyanobacteria to higher plants. In this study, reconstitution experiments were performed for the extrinsic 23 and l7kDa proteins from spinach with spinach PSII which had been exchanged for the 33kDa protein from different sources. Spinach PSII was treated with 2. 6 M urea/0. 2 M NaCl to remove all of the extrinsic proteins, and then reconstituted with the 33kDa protein from a cyanobacterium (Synechococcus vulcanus), a red alga (Gyanidium caldarium), a higher plant (Spinach). The results showed that spinach PSII which had been exchanged for the red algal or cyanobacterial 33kDa protein partially rebound the 23kDa protein but totally failed to bind the l7kDa protein, although the PSII bearing its own 33kDa protein retained the full ability to bind the 23 and l7kDa proteins. This suggests that the 33kDa protein has a different structure between higher plants and red algae or cyanobacteria, at least from the viewpoint of its interaction with other extrinsic proteins.