AlgK is an outer-membrane lipoprotein involved in the biosynthesis of alginate in Pseudomonads and Azotobacter vinelandii. A recombinant form of Pseudomonas fluorescens AlgK with a C-terminal polyhistidine affinity tag has been expressed and purified from the periplasm of Escherichia coli cells and diffraction-quality crystals of AlgK have been grown using the hanging-drop vapour-diffusion method. The crystals grow as flat plates with unit-cell parameters a = 79.09, b = 107.85, c = 119.15 Å, β = 96.97°. The crystals exhibit the symmetry of space group P21 and diffract to a minimum d-spacing of 2.5 Å at Station X29 of the National Synchrotron Light Source, Brookhaven National Laboratory. On the basis of the Matthews coefficient (V M = 2.53 Å3 Da−1), four protein molecules are estimated to be present in the asymmetric unit.