Mannosylglycerate (MG) is a compatible solute that is widespread in marine organisms that are adapted to hot environments, with its intracellular pool generally increasing in response to osmotic stress. These observations suggest that MG plays a relevant role in osmoadaptation and thermoadaptation. The pathways for the synthesis of MG have been characterized in a number of thermophilic and hyperthermophilic organisms. Mannosyl-3-phosphoglycerate synthase (MpgS) is a key enzyme in the biosynthesis of MG. Here, the purification, crystallization and preliminary crystallographic characterization of apo MpgS from Thermus thermophilus HB27 are reported. The addition of Zn(2+) to the crystallization buffer was essential in order to obtain crystals. The crystals belonged to one of the enantiomorphic tetragonal space groups P4(1)2(1)2 or P4(3)2(1)2, with unit-cell parameters a = b = 113, c = 197 A. Diffraction data were obtained to a resolution of 2.97 A.