Histidine kinases are part of the two-component signal transduction system responsible for eubacterial responses to diverse environmental signals. They have recently been detected in eukaryotes but their existence in the kingdom Archaea remains uncertain. Here we report the sequence and function of a histidine kinase (CheAH.s.) from Halobacterium salinarium, the first such transmitter in Archaea. The protein CheAH.s. (668 residues) has significant sequence identity with the CheA proteins known from eubacterial signal transduction (e.g. 34% identity with CheA from Bacillus subtilis). Antibodies were raised against CheAH.s. as expressed in Escherichia coli and were used in Western blotting to demonstrate the expression of cheAH.s. in H. salinarium. As has been observed for other halophilic proteins, CheAH.s. has a deviant electrophoretic migration, with an apparent molecular weight of 103 kDa on SDS-PAGE compared with a calculated molecular weight of 72 kDa. Deletion of a part of the cheAH.s. gene leads to loss of both chemotactic and phototactic responses in H. salinarium as measured by swarm plate assays, motion analysis and tethering experiments. This indicates that CheAH.s. plays a crucial role in chemical and light signal integration, presumably interacting with at least two phototransducers and a number of chemoreceptors.