Glutamate mutase [inline image(B12)1] was reconstituted by incubating purified components E (inline image) and S (inline image) from Clostridium cochlearium, both produced in Escherichia coli, with either aquo- or cyanocobalamin. The inactive glutamate mutase obtained was crystallized with polyethyleneglycol 4000 as precipitant. Crystals are monoclinic with space group P21 and have cell dimensions a = 64.6, b = 113.2, c = 108.4 Å and β = 96.0° for the glutamate mutase reconstituted with aquocobalamin. They diffract to a resolution of at least 2.7 Å. Isolated component S was crystallized in the presence of an excess of cyanocobalamin, yielding red crystals of space group I422 with unit-cell dimensions of a = b = 69.9 and c = 107.1 Å. The crystals diffract to about 3.2 Å resolution. Native data sets were collected for both crystal forms.