The structure of the phosphoglycerate kinase (PGK) from Bacillus stearothermophilus, a moderate thermophile, has been determined and compared with that of its mesophilic equivalent from yeast. The Bacillus enzyme structure was solved by molecular replacement and improved using constrained rigid-body, molecular dynamics and conventional refinement procedures. The refinement residual, calculated using all the measured data between 8 and 1.65 A, is 0.18(1). The stereo chemical deviations of the final model from ideality are 0.01 A for both bonds and planes. The mid-point temperatures of the Bacillus and yeast enzymes are 67 and 53 degrees C, respectively. Differential scanning calorimetry indicates that the energy difference (delta delta G) between the mesophilic and thermophilic enzymes is of the order of 5 kcal mol-1 at room temperature. The structure comparison indicates that the features most likely to be responsible for the increased thermal stability of the Bacillus enzyme are the increased internal hydrophobicity, additional ion pairs, and better alpha-helix stability resulting from the removal of helix destabilizing residues and extra helix-dipole/helix side chain ionic interactions.