The supramolecular organization of photosystem II (PSII) complexes in the photosynthetic membrane of the cyanobacterium Synechocystis 6803 was studied by electron microscopy. After mild detergent solubilization, crystalline PSII arrays were extracted in which dimeric PSII particles associate in multiple rows. Image processing of the arrays shows that the PSII dimers are tightly packed at distances of 12.2 and 16.7 nm. The domains are considered to be an important type of association for preventing either spill-over energy from PSII towards photosystem I (PSI) or direct energy flow from phycobilisomes to PSI, because the latter can only be at periphery of the arrays.