Abstract Photosystem (PS) I preparations from spinach and from a cyanobacterium contain two molecules of phylloquinone (2-methyl-3-phytyl-1,4-naphthoquinone) per photosystem, only one of which is rapidly destroyed by ultraviolet light. In preparations of the core of PS I, the P700 reaction center, both quinone molecules are rapidly destroyed by UV irradiation. Nearly complete destruction does not alter the shape and size of the EPR spectrum of the electron acceptor A 1, which is believed to be phylloquinone. Photooxidation of P700, the primary electron donor of PS I, is also not affected. The results provide strong evidence against the identity of redox center A 1 with the naphthoquinone.