Retinal-containing proteins of archaea comprise a single family of homologous proteins that fall into three clusters correlating with function: the proton-transporting bacteriorhodopsins, the chloride-transporting halorhodopsins and the colour-discriminating sensory rhodopsins. Statistical and phylogenetic analyses, a multiple alignment and average hydropathy and similarity plots of these protein sequences are presented. Available evidence suggests that sequence conservation generally correlates with functional significance. Little or no evidence substantiates the proposal that these proteins arose by a tandem intragenic duplication event. The bacterial rhodopsin family appears to have evolved from a common ancestor without recognizable intragenic rearrangements.