1. 1. A spectral analysis of six mutant strains of the unicellular alga, Chlamydomonas reinhardti, having lesions associated with the water-splitting oxygen-evolving apparatus is described. 2. 2. The chloroplast fragments from these mutant strains have a functional Photosystem II reaction center as shown by the normal photoreduction of C-550 and the photooxidation of cytochrome b559 at low temperature, and by their ability to carry out a 3-(3,4-dichlorophenyl)-1, 1-dimethylurea-sensitive photoreduction of NADP+ at room temperature in the presence of electron donors which donate electrons specifically to Photosystem II. 3. 3. The mutant chloroplast fragments contain equal amounts of the ascorbate-reducible cytochrome b559 and C-550 with all of the cytochrome b559 present participating in the low-temperature photoreaction. In contrast the wild-type chloroplast fragments contain twice the amount of high-potential cytochrome b559 as C-550 with only half of the cytochrome b559 present participating in the low-temperature photoreaction. 4. 4. The only component difference found between the mutant and the wild-type chloroplast fragments was that the lfd chloroplast fragments contained only half as much ascorbate-reducible cytochrome b559 as the wild type. Equal proportions, respectively, of cytochrome c553, low-potential cytochrome b559, and cytochrome b564 are found in both the mutants and wild-type strains. 5. 5. The data indicate that there exist two functionally distinct pools of ascorbate-reducible cytochrome b559 both of which function in the water-splitting oxygen-evolving apparatus. The lfd mutant strains are unable to split water and evolve oxygen because they lack one of the two pools of ascorbate-reducible cytochrome b559.