Cytokinins are a class of plant hormones that are central to the regulation of cell division and differentiation in plants. It has been proposed that they are detected by a two-component system, because overexpression of the histidine kinase gene CKI1 induces typical cytokinin responses and genes for a set of response regulators of two-component systems can be induced by cytokinins. Two-component systems use a histidine kinase as an environmental sensor and rely on a phosphorelay for signal transduction. They are common in microorganisms, and are also emerging as important signal detection routes in plants. Here we report the identification of a cytokinin receptor. We identified Arabidopsis cre1 (cytokinin response 1) mutants, which exhibited reduced responses to cytokinins. The mutated gene CRE1 encodes a histidine kinase. CRE1 expression conferred a cytokinin-dependent growth phenotype on a yeast mutant that lacked the endogenous histidine kinase SLN1 (ref. 10), providing direct evidence that CRE1 is a cytokinin receptor. We also provide evidence that cytokinins can activate CRE1 to initiate phosphorelay signalling.