'Squid-type' diisopropylfluorophosphatases (DFPases), a subclass of the phosphotriesterases, are enzymes capable of hydrolysing organophosphorus nerve agents. To date, no three-dimensional structure of a 'squid-type' DFPase is known. Here, the crystallization of the DFPase originally isolated from head ganglion of the squid Loligo vulgaris is reported. The protein has been heterologously expressed in Escherichia coli, purified to homogeneity and subsequently crystallized. The protein crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 43.1, b = 82.1, c = 86.6 A and one monomer per asymmetric unit. Under cryoconditions (120 K) the crystals diffracted beyond 2.0 A using a Cu rotating-anode X-ray generator.