Cytochrome c-552 from Nitrosomonas europaea was crystallized by the sandwiched drop, vapor diffusion method, with anmmonium sulfate as the precipitant. The crystals were found to belong to the space group P2(1)2(1)2(1), having unit cell dimensions of a = 106.1 A, b = 126.1 A, and c = 57.7 A. The crystals diffracted X-rays at greater than 3.0 A resolution.