Manganese superoxide dismutase (GP-MnSOD), a component of the so-called 'green protein' (green protein complex) from the facultative anaerobic halodenitrifier Bacillus halodenitrificans, has been crystallized using the hanging-drop vapor diffusion method. Crystals have unit-cell parameters a = b = 93.4 Angstrom, c = 65.0 Angstrom, and belong to the space group P4(3)2(1)2. Preliminary analysis indicates there is one monomer in each asymmetric unit. The structural information from this enzyme will enrich our knowledge on its high catalytic activity and its possible role in green protein complex. (C) 2002 Elsevier Science (USA). All rights reserved.