Crystals of the apoenzyme of tyrosine phenol-lyase (EC 4.1.99.2), a pyridoxal 5'-phosphate-dependent enzyme from Citrobacter intermedius, have been grown by vapor diffusion of an ammonium sulfate solution to a protein solution. The crystals belong to space group P2(1)2(1)2, with dimensions of a = 75.5 A, b = 138.4 A and c = 94.1 A and diffract up to 2.7 A resolution. The asymmetric unit contains one half of the enzyme tetrameric molecule. Two heavy-atom derivatives of the crystals have been obtained.