Apo-salicylate hydroxylase from Pseudomonas putida S-1 has been crystallized by the dialysis method, using ammonium sulfate as the precipitant. The crystals belong to hexagonal space group P6(2) or P6(4) with unit cell dimensions of a = b = 142.8 A and c = 63.8 A, and diffract X-rays at higher than 3.5 A resolution. A heavy-atom derivative has been prepared by soaking a crystal in an ammonium sulfate solution containing p-chloromercuriphenylsulfonate.