Photosystem II phycobilisome particles were isolated from the red alga Porphyridium cruentum in a sucrose-phosphate-citrate-magnesium medium. These particles showed high rates of oxygen evolution in the water-to-silicomolybdate reaction (2300 μmol O 2 .mg chl - 1 .h - 1 ). Photosystem I, according to fluorescence emission spectra (-196 o C), was greatly reduced or absent. The results demonstrate a close physical relationship of photosystem II with the phycobilisomes which are the major light harvesting antennae in this organism.