The three-dimensional structure of the light-harvesting pigment-protein b-phycoerythrin from the red alga Porphyridium cruentum has been determined at 0.23-nm resolution. The b-phycoerythrin structure is very similar to the structure of B-phycoerythrin from Porphyridium sordidum. Besides three non-identical residues there are only small differences between b-phycoerythrin and B-phycoerythrin α and β subunits, respectively. In the crystals b-phycoerythrin forms an (αβ)6 hexamer (molecular mass: 236 kDa), whereas B-phycoerythrin additionally contains a 30-kDa γ subunit. The comparison of the b-phycoerythrin and B-phycoerythrin electron-density maps clearly reveals, that the γ subunit is located inside the (αβ)6 aggregate.