In the archaebacterium Halobacterium halobium membrane proteins have evolved that are similar in many respects to the visual pigment rhodopsin found in multicellular eukaryotes. The structure and function of these archaebacterial retinal-protein pigments will be described as well as their relationship to the eukaryotic rhodopsins. Evolutionary connections between different rhodopsins are discussed since this knowledge might contribute to our understanding of early cellular evolution.