Cuticle-degrading proteases are extracellular subtilisin-like serine proteases that are secreted by entomopathogenic and nematophagous fungi. These proteases can digest the host cuticle during invasion of an insect or nematode and serve as a group of important virulence factors during the infection of nematodes by nematophagous fungi. To elucidate the mechanism of interaction between the proteases and the nematode cuticle, two cuticle-degrading proteases, Ver112 from Lecanicillium psalliotae (syn. Verticillium psalliotae) and PL646 from Paecilomyces lilacinus, were studied. The Ver112 protein and the complex between PL646 and the substrate-like tetrapeptide inhibitor methoxysuccinyl-Ala-Ala-Pro-Val-chloromethyl ketone (MSU-AAPV) were crystallized using the hanging-drop vapour-diffusion method at 289 K. The crystals were analyzed by X-ray diffraction to resolutions of 1.65 and 2.2 A, respectively. These analyses identified that crystals of Ver112 belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 43.7, b = 67.8, c = 76.3 A, alpha = beta = gamma = 90 degrees . In contrast, crystals of the PL646-MSU-AAPV complex belonged to space group P2(1), with unit-cell parameters a = 65.1, b = 62.5, c = 67.6 A, beta = 92.8 degrees .