Crystals of rubredoxin oxygen oxidoreductase have been obtained and characterized. They belong to space group P21212, with unit-cell dimensions a = 88.24 (15), b = 101.25 (7), c = 90.80 (3) Å. The homodimer (86 kDa) in the asymmetric unit is related by a non-crystallographic twofold rotation axis parallel to the ab `diagonal' direction, as shown by the self-rotation maximum in the section with χ = 180°. This pseudo-crystallographic symmetry element was also found to be the twinning axis of pseudo-merohedrally twinned crystals, leading to apparent pseudo-tetragonal P4212 crystal symmetry.