Photoreduction of Pheophytin 'a' (Pheo) accompanied by a decrease in the chlorophyll fluorescence yield is observed in photosystem II (PS II) of the whole cells of green algae Chlamydomonas reinhardii (a wild type and a mutant lacking both photosystem I and chlorophyll 'b'), Chlorella pyrenoidosa, Scenedesmus obliquus and cyanobacteria Phormidium laminosum, Anabaena variabilis and Cynechococcus elongatus under anaerobic conditions created by means of the glucose-glucoseoxidase-catalase. The photoreaction is activated by the addition of 1 ΞΌM CCCP, inhibited by 10 ΞΌM DCMU and reactivated upon subsequent addition of either ascorbate or dithionite. Oxidized NADP, benzyl viologen and methyl viologen accelerate dark oxidation of the reduced Pheo indicating that they are able to accept an electron from [Formula: see text] in PS II.The data on both photoreduction of Pheo in the intact cells in the absence of exogenous reductants, when electron donation to reaction centers of PS II occurs only from water, and the inhibition of this photoreaction by DCMU, show that the Pheo photoreduction is sensitized by the reaction centers of PS II and probably occurs as a result of the electron donation from the water-splitting system being in the state S3, to [Formula: see text] producing the long-lived state [Formula: see text] and O2.