The structure of photosystem II (PSII) complex isolated from thylakoid membranes of the red alga Porphyridium cruentum was investigated using electron microscopy fol-lowed by single particle image analysis. The dimeric com-plexes observed contain all major PSII subunits (CP47, CP43, D1 and D2 proteins) as well as the extrinsic proteins (33 kDa, 12 kDa and the cytochromec550 ) of the oxygen-evolvingcomplex (OEC) of PSII, encodedby thepsbO,psbU andpsbVgenes, respectively.