Optical properties of carotenoids in photoreceptor systems for vision, phototaxis and phototropism are studied by LCAO-ASMO-SCF-CI theory, using ZDO approximation for the π-electrons and CNDO/2 method for the valence-shell electrons. The most important point of our π-electron approximation is that the σ-π interactions indispensable to derive the bond alternation are taken into account explicitly. The conclusions are as follows: (1) the wavelengths of absorption maximum (λmax) calculated as the function of the number of double bonds are brought into a very satisfactory agreement with the experiments; (2) the wide distribution of the λmax's of known visual pigments is reasonably explained by the protonated Schiff-base binding of retinal with opsin; (3) the cis-trans isomerization is non-adiabatic transition, at which the intersection of adiabatic potentials between a singlet and a triplet state plays an important role; (4) the origin of the cis-peals is the steric hindrance of a hydrogen to a methyl branch or another hydrogen.