C-phycocyanin and allophycocyanin from the green alga Spirulina platensis were isolated and crystallized by gel-acupuncture techniques. A novel two-step chromatographic procedure was used for purification. Blue hexagonal crystals were obtained by diffusing magnesium chloride into the protein solution for a week, followed by diffusion of PEG 6000 in order to complete the reduction of the solubility of the protein in the capillary tube used as a growth cell. In the case of allophycocyanin, crystals with a size of 0.4 × 0.3 × 0.3 mm were characterized by X-ray diffraction. They belong to space group P6322 with unit-cell parameters a = b = 102.04, c = 131.22 Å. The crystals of C-phycocyanin belong to either space group P6 or P63 with unit-cell constants a = b = 182.38, c = 60.87 Å, α = β = 90, γ = 120°. The crystals diffract beyond 2.4 and 2.5 Å resolution, respectively, using a rotating anode as an X-ray source.