AlgX is a periplasmic protein required for the production of the exopolysaccharide alginate in Pseudomonas sp. and Azotobacter vinelandii. AlgX has been overexpressed and purified and diffraction‐quality crystals have been grown using iterative seeding and the hanging‐drop vapor‐diffusion method. The crystals grew as flat plates with unit‐cell parameters a = 46.4, b = 120.6, c = 86.9 Å, β = 95.7°. The crystals exhibited the symmetry of space group P21 and diffracted to a minimum d‐spacing of 2.1 Å. On the basis of the Matthews coefficient (VM = 2.25 Å3 Da−1), two molecules were estimated to be present in the asymmetric unit.