The primary electron donor of photosystem (PS) II in the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina was confirmed by delayed fluorescence (DF) and further proved by pigment contents of cells grown under several light intensities. The DF was found only in the Chl a region, identical to Synechocystis sp. PCC 6803, and disappeared following heat treatment. Pigment analyses indicated that at least two Chl a molecules were present per each two pheophytin a molecules, and these Chl a molecules are assigned to P(D1) and P(D2). These findings clearly indicate that Chl a is required for water oxidation in PS II.