Two different crystal forms of cytochrome c from the yeast Valida membranaefaciens have been grown by vapor diffusion from polyethylene glycol 4000. Both have been characterized by X-ray diffraction. The first crystal form was grown at 277 K but proved unstable and dissolved when the temperature was increased above 288 K. This crystal was of triclinic space group P1 and had unit-cell dimensions of a = 68.4, b = 35.7, c = 33.24 A, alpha = 116.9, beta = 114.9, gamma = 75.0 degrees. Following dissolution of the triclinic crystals, the second crystal form also grew at 277 K but remained stable even at room temperature. It was of orthorhombic space group P2(1)2(1)2(1) with cell dimensions a = 27.95, b = 63.68, and c = 64.28 A. Although rather small in size, both crystal forms diffracted X-rays well and the resolution of the patterns extended beyond 3 A.