Laminarinase endo-beta-1,3 glucanase (LamA) from Pyrococcus furiosus is an enzyme which displays its main hydrolytic activity on the 3-1,3-glucose polymer laminarin. This laminarinase is remarkably resistant to denaturation: its secondary structure is unchanged in 8 M guanidinium chloride. This protein belongs to the family 16 glycosyl hydrolases, which are enzymes that are widely distributed among bacteria, fungi and higher plants. Single crystals of P. furiosus LamAhave been obtained by the hanging-drop vapour-diffusion method using 2-methyl-2,4-pentanediol as a precipitant agent. A complete data set has been collected under cryocooling at a synchrotron source. The crystals belong to the monoclinic space group P21, with unit-cell parameters a = 44.36, b = 84.76, c = 69.23 A, a = 90, fl = 104.97, y = 90 degrees, and diffract to 2.15 A resolution.