The genome of the halophilic archaeon Halobacterium salinarum encodes the high-affinity ATP-dependent K(+) uptake system Kdp. Previous studies have shown that the genes coding for the KdpFABC complex are arranged in a kdpFABCQ gene cluster together with an additional gene kdpQ. In bacteria, expression of the kdpFABC genes is generally regulated by the dedicated sensor kinase/response regulator pair KdpD/KdpE, which are absent in H. salinarum. Surprisingly, H. salinarum expresses the kdp genes in a manner which is strikingly similar to Escherichia coli. In this study, we show that the halobacterial kdpFABCQ genes constitute an operon and that kdpFABCQ expression is subject to a complex regulatory mechanism involving a negative transcriptional regulator and is further modulated via a so far unknown mechanism. We describe how the regulation of kdp gene expression is facilitated in H. salinarum in contrast to its bacterial counterparts. Whereas the Kdp system fulfils the same core function as an ATP-driven K(+) uptake system in both archaea and bacteria, the different mechanisms involved in the regulation of gene expression appear to have evolved separately, possibly reflecting a different physiological role of ATP-driven K(+) uptake in halophilic archaea.