Helicobacter pylori infection of the stomach can lead to severe gastroduodenal diseases such as gastritis, peptic ulcers and gastric cancers. Periplasmic H. pylori -carbonic anhydrase (HpCA) is essential for the acclimatization of the bacterium to the acidity of the stomach. Through the action of urease and carbonic anhydrases, the H. pylori periplasmic pH is maintained at around 6 in an environment with a pH as low as 2, which in turn facilitates the maintenance of a cytoplasmic pH close to neutral, allowing growth in the gastric niche. Crystals of HpCA in complex with the inhibitor acetazolamide have been grown by the hanging-drop vapour-diffusion method using polyethylene glycol as a precipitating agent. The crystals have the symmetry of space group P212121, with unit-cell parameters a = 37.0, b = 82.4, c = 150.8Å. An X-ray diffraction data set was collected from a single crystal to 1.7Å resolution. Calculation of the self-rotation function using this data and molecular replacement showed that the asymmetric unit contains an HpCA dimer. © 2013 International Union of Crystallography All rights reserved.