Purple acid phosphatase from sweet potato is a homodimer of 110 kDa. Two forms of the enzyme have been characterized. One contains an Fe-Zn centre similar to that previously reported for red kidney bean purple acid phosphatase. Another isoform, the subject of this work, is the first confirmed example of an Fe-Mn-containing enzyme. Crystals of this protein have been grown from PEG 6000. They have unit-cell parameters a = b = 118.4, c = 287.4 A and have the symmetry of space group P6(5)22, with one dimer per asymmetric unit. Diffraction data collected using a conventional X--ray source from a cryocooled crystal extend to 2.90 A resolution. The three-dimensional structure of the enzyme will provide insight into the coordination of this novel binuclear metal centre.