BphA3, a Rieske-type [2Fe-2S] ferredoxin component of a biphenyl dioxygenase (BphA) from Pseudomonas sp. strain KKS102, was crystallized by the hanging-drop vapour-diffusion method. Two crystal forms were obtained from the same conditions. The form I crystal belongs to space group P2(1)2(1)2, with unit-cell parameters a = 26.3, b = 144.3, c = 61.5 A, and diffracted to 2.45 A resolution. The form II crystal belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 26.2, b = 121.3, c = 142.7 A, and diffracted to 2.8 A resolution. A molecular-replacement calculation using BphF as a search model yielded a satisfactory solution for both forms.