The crystal structure of C-phycocyanin, a light-harvesting phycobiliprotein from cyanobacteria (blue-green algae) Spirulina platensis has been solved by molecular replacement technique. The crystals belong to space group P2(1) with cell parameters a 107.20, b = 115.40, c = 183.04 Angstrom; beta = 90.2 degrees. The structure has been refined to a crystallographic R factor of 19.2% (R-free = 23.9%) using the X-ray diffraction data extending up to 2.2 Angstrom resolution. The asymmetric unit of the crystal cell consists of two (alpha beta)(6)-hexamers, each hexamer being the functional unit in the native antenna rod of cyanobacteria. The molecular structure resembles that of other reported C-phycocyanins. However, the unique form of aggregation of two (alpha beta)(6)-hexamers in the crystal asymmetric unit, suggests additional pathways of energy transfer in lateral direction between the adjacent hexamers involving beta 155 phycocyanobilin chromophores.