Phototaxis in the archaeon Halobacterium salinarium is mediated by a stable complex of the photoreceptor sensory rhodopsin I and its transducer HtrI, which relays the light stimulus to the signalling pathway. Removal of the cytoplasmic signalling domain of HtrI eliminated the SRI-specific motor response to light stimulation and led to the loss of the spectroscopically detectable physical interaction of SRI and HtrI. A similar phenotype was obtained by deleting part of a cytoplasmic loop located between the second transmembrane helix of HtrI and the signalling domain. These results indicate that the photochemical behavior of sensory rhodopsin I is not determined by interaction with the transmembrane helices of HtrI per se but functionally coupled to the signalling domain. It is proposed that light excitation of SRI results in a conformational change of the transducer which is conducted by the cytoplasmic loop, an extra module not found in the eubacterial transducer homologues, and activates the signalling domain.