The trimeric signal-transduction protein GlnK, from Escherichia coli, has been over-expressed, purified to homogeneity and crystallized. The crystals belong to space group P213 with a = 85.53 Å and have two subunits in the asymmetric unit. The complex of GlnK with ATP crystallized in space group P63 with a = 57.45 Å and c = 54.79 Å. These crystals have a single subunit in the asymmetric unit. High-quality diffraction data from crystals of GlnK and the GlnK complex have been collected to 2.0 Å.