Allophycocyanin from red alga Porphyra yezoensis has been crystallized in three crystal forms. Form 1 crystals with space group P4132 or P4332 and cell parameters a = 286 A, alpha = 90 degrees were obtained by using isopropanol as precipitant. These crystals did not diffract beyond 5.8 A and could not be used in structure analysis. Form 2 crystals were obtained when crystallization conditions were slightly changed by adding 0.2 M magnesium chloride. The space group of form 2 was not determined because it was difficult to get large single crystals. Form 3 crystals were obtained by using ammonium sulfate as precipitant. The space group of form 3 is R32 with cell dimensions a = b = 105.3, c = 189.4 A and one alpha beta unit in the asymmetric unit. These crystals diffract up to 2.06 A resolution and are suitable for structure determination by molecular-replacement methods.