The receiver domain (RD) of a sensor histidine kinase (HK) catalyses the transphosphorylation reaction during the action of HKs in hormonal and abiotic signalling in plants. Crystals of the recombinant RD of the Arabidopsis thaliana HK CYTOKININ-INDEPENDENT1 (CKI1RD) have been obtained by the hanging-drop vapour-diffusion method using ammonium sulfate as a precipitant and glycerol as a cryoprotectant. The crystals diffracted to approximately 2.4 A resolution on beamline BW7B of the DORIS-III storage ring. The diffraction improved significantly after the use of a non-aqueous cryoprotectant. Crystals soaked in Paratone-N diffracted to at least 2.0 A resolution on beamline BW7B and their mosaicity decreased more than tenfold. The crystals belonged to space group C2221, with unit-cell parameters a = 54.46, b = 99.82, c = 79.94 A. Assuming the presence of one molecule of the protein in the asymmetric unit gives a Matthews coefficient VM of 2.33 A3 Da-1. A molecular-replacement solution has been obtained and structure refinement is in progress.