When membranes of the wild type strain of the cyanobacterium Synechocystis sp. PCC 6803 were solubilized with detergents and fractionated by sucrose-gradient ultracentrifugation, photosystem I could be obtained as trimers and monomers. We could not obtain trimers from the membranes of any mutant strain that lacked PsaL subunit. In contrast, absence of PsaE, PsaD, PsaF, or PsaJ did not completely abolish the ability of photosystem I to form trimers. Furthermore, PsaL is accessible to digestion by thermolysin in the monomers but not in the trimers of photosystem I purified from wild type membranes. Therefore, PsaL is necessary for trimerization of photosystem I and may constitute the trimer-forming domain in the structure of photosystem I.