Dissimilatory sulfite reductase D (DsrD) from Desulfovibrio vulgaris has been crystallized for a neutron diffraction study. The initial crystals obtained were too small for the neutron experiment. In order to obtain a larger crystal (>1 mm3), a combination of two techniques was developed to determine the optimum crystallization conditions: a crystallization phase diagram was obtained, followed by crystal-quality assessment via X-ray diffraction. Using conditions determined in this manner, a large single crystal (1.7 mm3) of DsrD protein was subsequently grown in D(2)O solution by the macroseeding technique. A neutron diffraction experiment was carried out using the BIX-3 diffractometer at the Japan Atomic Energy Research Institute (JAERI), collecting data to 2.4 A resolution from an optimized crystal.