The enzymatic formation of glycosidic bonds may be catalyzed by the transfer of the glycosyl moiety from an activated nucleotide-diphospho-sugar donor to a specific acceptor. SpsA is a glycosyltransferase implicated in the synthesis of the spore coat of Bacillus subtilis, whose homologues include cellulose synthase and many lipopolysaccharide and bacterial O-antigen synthases. The three-dimensional crystal structure of SpsA has been determined by conventional MIR techniques at a resolution of 1.5 A. It is a two-domain protein with a nucleotide-binding domain together with an acceptor binding domain which features a disordered loop spanning the active site. The structures of SpsA in complex with both Mg-UDP and Mn-UDP have also been determined at 2.0 and 1.7 A, respectively. These complexes, together with the sequence conservation, begin to shed light on the mechanism of this ubiquitous family of inverting glycosyltransferases.