Ferredoxin-NADP(H) reductase (272 amino acids) from Rhodobacter capsulatus (FPR) has recently been postulated to be involved in the antioxidant response and to facilitate the provision of reduced flavodoxin for the reduction of nitrogenase. Crystallization trials of recombinant FPR were carried out at 291 K by the hanging-drop vapour-diffusion method. Orthorhombic crystals (unit-cell parameters a = 69.3, b = 93.6, c = 103.5 A) were obtained. However, their diffraction pattern was not satisfactory and an extensive detergent screening was carried out over the initial crystallization conditions. The introduction of n-heptyl-beta-D-thioglucoside produced new trigonal crystals (space group P3(1)21; unit-cell parameters a = b = 120.5, c = 51.1 A) that diffracted to 1.8 A resolution at beamline BM16 at the ESRF. Preliminary structural analysis indicated that detergent molecules could increase the quality of diffraction of FPR crystals by stabilizing the disordered regions of the protein and by increasing the number of contacts in the crystal packing.