Summary: The in situ location of phytoene desaturease, an enzyme in the carotenoid biosynthetic pathway, has been investigated in the photosynthetic organisms Rhodobacter capsulatus SB 1003, Synechocystis PCC 6714, and Anabaena variabilis ATCC 29413. A post-embedding immunogold electyron microscopy procedure using a polyclonal antibody against a phytoene desaturase fusion protein was employed. For all three species most of the gold particles (about 85% in the case of Anabaena and Synechocystis) were associated with the photosynthetic membreanes. Other parts of the were only randomly and not significanlty labelled. When the anti-phytoene desatuase antibody was omitted, or when pre-immune serum was used, no gold deposition was observed. In vitro phytoene desaturase reactions, especially with purified thylakoid membranes from Synechocystis, confirmed the nature of phytoene as an integral protein of the thylakoid membrane.