Structure and organisation of Photosystem I and Photosystem II isolated from red alga Cyanidium caldarium was determined by electron microscopy and single particle image analysis. The overall structure of Photosystem II was found to be similar to that known from cyanobacteria. The location of additional 20 kDa (PsbQ′) extrinsic protein that forms part of the oxygen evolving complex was suggested to be in the vicinity of cytochrome c-550 (PsbV) and the 12 kDa (PsbU) protein. Photosystem I was determined as a monomeric unit consisting of PsaA/B core complex with varying amounts of antenna subunits attached. The number of these subunits was seen to be dependent on the light conditions used during cell cultivation. The role of PsaH and PsaG proteins of Photosystem I in trimerisation and antennae complexes binding is discussed.