The Escherichia coli aceK gene encodes isocitrate dehydrogenase kinase/phosphatase (EC 18.104.22.168), a bifunctional protein that phosphorylates and dephosphorylates isocitrate dehydrogenase (IDH), resulting in its inactivation and activation, respectively. This reversible (de)phosphorylation directs isocitrate, an intermediate of the citric acid cycle, to either go through the full cycle or to enter the glyoxylate bypass. In the present study, the AceK protein from E. coli has been purified and crystallized. Three crystal forms were obtained from very similar crystallization conditions. The crystals belong to space groups P4(1)2(1)2, P3(2)21 and P2(1)2(1)2(1) and diffracted X-rays to resolutions of 2.9, 3.0 and 2.7 A, respectively.