Archael-type rhodopsin has an all-trans or a 13-cis retinal. The light-induced interconversion between these two forms has been found in Anabaena sensory rhodopsin, even though only the photoreaction from the 13-cis form to the all-trans form exists in bacteriorhodopsin. In this study, we obtained the quantum yields for the 13-cis all-trans and all-trans 13-cis reactions of Anabaena sensory rhodopsin (0.24+/-0.03 and 0.38+/-0.07, respectively) and concluded that these values were independent of the wavelength of the excitation light as well as bacteriorhodopsin. In other words, no excess energy effects can be found in these reactions. (C) 2008 Published by Elsevier B.V.