The x-ray structure of the lipase fromPseudomonas aeruginosa PAO1 has been determined at 2.54 Å resolution. It is the first structure of a member of homology family I.1 of bacterial lipases. The structure shows a variant of the α/β hydrolase fold, with Ser⁸², Asp²²⁹, and His²⁵¹ as the catalytic triad residues. Compared with the “canonical” α/β hydrolase fold, the first two β-strands and one α-helix (αE) are not present. The absence of helix αE allows the formation of a stabilizing intramolecular disulfide bridge. The loop containing His²⁵¹ is stabilized by an octahedrally coordinated calcium ion. On top of the active site a lid subdomain is in an open conformation, making the catalytic cleft accessible from the solvent region. A triacylglycerol analogue is covalently bound to Ser⁸² in the active site, demonstrating the position of the oxyanion hole and of the three pockets that accommodate thesn-1, sn-2, and sn-3 fatty acid chains. The inhibited enzyme can be thought to mimic the structure of the tetrahedral intermediate that occurs during the acylation step of the reaction. Analysis of the binding mode of the inhibitor suggests that the size of the acyl pocket and the size and interactions of the sn-2 binding pocket are the predominant determinants of the regio- and enantio-preference of the enzyme.