The moderate thermophilic bacterium Bacillus stearothermophilus P1 expresses a thermostable lipase that was active and stable at the high temperature. Based on secondary structure predictions and secondary structure-driven multiple sequence alignment with the homologous lipases of known three-dimensional (3-D) structure, we constructed the 3-D structure model of this enzyme and the model reveals the topological organization of the fold, corroborating our predictions. We hypothesized for this enzyme the α/β-hydrolase fold typical of several lipases and identified Ser-113, Asp-317, and His-358 as the putative members of the catalytic triad that are located close to each other at hydrogen bond distances. In addition, the strongly inhibited enzyme by 10 mM PMSF and 1-hexadecanesulfonyl chloride was indicated that it contains a serine residue which plays a key role in the catalytic mechanism. It was also confirmed by site-directed mutagenesis that mutated Ser-113, Asp-317, and His-358 to A1a and the activity of the mutant enzyme was drastically reduced. © 2001 Academic Press.