AbstractThermitase is a thermostable member of the subtilisin family of serine proteases. Four independently determined crystal structures of the enzyme are compared in this study: a high resolution native one and three medium resolution complexes of thermitase with eglin‐c, grown from three different calcium concentrations. It appeared that the B‐factors of the thermitase eglin complex obtained at 100 mM CaCl2and elucidated at 2.0 Å resolution are remarkably similar to those of the 1.4 Å native structure: the main chain atoms have an rms difference of only 2.3 Å2; for all atoms this difference is 4.6 Å2. The rms positional differences between these two structures of thermitase are 0.31 Å for the main chain atoms and 0.58 Å for all atoms. There results show that not only atomic positions but also temperature factors can agree well in X‐ray structures determined entirely independently by procedures which differ in virtually every possible technical aspect.A detailed comparison focussed on the effects of eglin binding on the structure of thermitase. Thermitase can be considered as consisting of (1) a central core of 94 residues, plus (2) four segments of 72 residues in total which shift as rigid bodies with respect to the core, plus (3) the remaining 113 residues which show small changes but, however, cannot be described as rigid bodies. The central cores of native thermitase and the 100 mM CaCl2thermitase:eglin complex have an rms deviation of 0.13 Å for 376 main chain atoms. One of the segments, formed by loops of the strong calcium binding site, shows differences up to 1.0 Å in Cαpositions. These are probably due to crystal packing effects.The three other segments, comprising 51 residues, are affected conformational changes upon eglin binding so that the P1 to P3 binding pockets of thermitase broaden by 0.4 to 0.7 Å. The residues involved in these changes correspond with residues which change position upon inhibitor binding in other subtilisins. This suggests that an induced fit mechanism is operational during substrate recognition