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The crystal structure of C-phycocyanin from the cyano-bacterium S. platensis has been determined at 2.2 AÊ resolution. The crystals belong to the monoclinic crystal form, which has not been previously reported for phycobili-protein structures. The structure was solved using the molecular-replacement method with a ®nal R value of 18.9 % (Rfree = 23.7%) after model building and re®nement. In the crystals used for the study, the C-phycocyanin hexamers formed by face-to-face association of two trimers are arranged in layers rather than in columns. Three different kinds of packing between adjacent hexamers in the layer were compared. The tight packing of two adjacent hexamers formed by four trimers in the asymmetric unit brings 155 PCB chromophores close together, so it is possible that lateral energy transfer takes place through the 155 ± 155 route