Thermostable monoacylglycerol lipase (MGLP; EC 3.1.1.23) from the moderately thermophilic Bacillus sp. H-257 has a unique substrate specificity. It hydrolyzes monoacylglycerols but does not hydrolyze di- or triacylglycerols. Crystals of the enzyme were obtained by the hanging-drop vapour-diffusion method using ammonium sulfate as a precipitant and benzamidine as an additive. The orthorhombic crystals belong to the space group P2(1)2(1)2(1), with unit-cell parameters a = 43.53, b = 100.82, c = 108.17 A. The crystals diffract to at least 2.3 A resolution and a native data set has been collected to 2.6 A resolution on a CCD detector using synchrotron radiation.