Summary Cytochrome b 559 (Cyt b 559) is an intrinsic and essential component of the photosystem II (PSII) reaction center, but its physiological function remains yet undefined. In this study, a partial redox characterization of Cyt b 559 in photosynthetic membranes from the transformable unicellular cyanobacterium Synecborystis sp. PCC 6803 (hereafter called Synecborystis 6803) has been performed. In thylakoid membranes extracted by a very mild cell-breaking procedure, as developed in this work, only one redox form of Cyt b 559 was found, with a redox potential proper to a low-potential (LP) form. The midpoint redox potential value of this LP form has been shown to be + 170 mV at pH 7.5 in PSII-enriched membranes. In spite of a great variety of cell-breaking treatments used to prepare thylakoid membranes, it has not been possible to detect any high-potential (HP) form of Cyt b 559. The absence of this HP form is discussed in terms of its higher lability in Synecborystis 6803 as compared with other cyanobacteria and higher plants. Alternatively, the possibility is considered that a HP form of Cyt b 559 does not occur in Synecborystis 6803 because it can not be properly stabilized in the membrane environment.