Single crystal x-ray diffraction studies are proceeding on two phycobiliproteins: C-Phycocyanin from Anabaena variabilis and B-Phycoerythrin from Porphyridium cruentum. C-Phycocyanin consists of six alpha and six beta subunits. A three-dimensional x-ray diffraction study of this light-harvesting protein (P6(3); a = b = 154.1 A, c = 40.1 A) at 5-A resolution shows that the molecule is 110 A in diameter, 40 A thick, with a 20-A diameter central channel. It can be divided into three separate domains related by the crystallographic 3 and contains long, columnar regions of density, presumed to represent alpha helix. B-Phycoerythrin is composed of 6 alpha, 6 beta, and a gamma subunit. A 5.25-A resolution study of this protein (R3, a = b = 189 A, c = 60 A) indicates that the molecule is 107 A in diameter and 55 A thick, has 32-D3 symmetry, and can be divided into six regions. The gamma chain lies on a crystallographic 3 and undergoes symmetric disordering. A region of low and unstructured density in the center of the molecule is presumably occupied by this gamma chains. These two low resolution structures are compared with regard to differences in quaternary structure.